Damn that is a dirty, shitty car. Hahah JK Luis! Let me shoot it too!
1. I like it. Good composition, decent lighting. Your color is a bit washed out, probably from a bit of over exposure.
To combat a lighting situation like this (hot lights on the hood, dark on the front end) take two exposures while using a tripod. Take one exposure that gives you good color and light for the hood, and one for the front end. Then in photoshop you can combine them in a number of ways, look up HDR images on google or you can layer them and erase way parts of whatever image you don't want.
Other than that, no complaints. Strong image.
2. Something's a bit off on this one. I like the bricks in the background but they aren't level, seems like it's two things, a non-level horizon, and lens distortion. You can't really help the lens distortion, but a more centric composition, and some leveling would help.
Try using the Measure tool in photoshop, draw two points with it along a straight line in your image, then goto edit>rotate canvas, and select the option "arbitrary". It will automatically rotate the image to level out whatever line you have drawn out with the measure tool. Then crop off the outside edges that will now be apparent.
This image isn't as strong, your subject just doesn't stand out as much due to lighting and size in the frame.
3. A cool composition, this is always an excellent angle to shoot on a car that is "flush".
To make it even stronger, park the car further from the surrounding walls, and zoom in as much as possible and (if you know how) use a "wide open aperture". If you don't know about aperture, now would be a good time for some Google. I could explain it but I'm sure there are millions of articles explaining shutter speed and aperture FAR better than I can in a little forum post.
Example (think of it reversed to match your positioning):
One thing that stands out is the orange piping. It just kind of breaks up the scene in a bad way, I find my eyes drawn to it. Another learning experience though, you can easily use the "clone stamp tool" here to get rid of it. The clone stamp tool is a photographers #1 BEST friend.
4. I see this angle a lot, and I like it, I like your background, and your lighting is cool (you could lighten up the car a little bit, try an HDR multiple exposure here too).
One major problem though is the car is cut off. I find myself wanting to see what the rest of the car looks like here. Use your widest angle possible, shoot low, and get the whole car in frame.
Example:

Shot at 18mm, tripod close to the ground. This way, when you're looking "up" at the vehicle, it gives the object "mass" it appears large and intimidating, like a car should.
5. Another good angle. You've definitely got the eye for this. These are almost text book good shots with just a few tweaks.
Main problem, too much space off to the side. The middle of the car should usually be present in this type of shot. Just feels easier on the eye.
The yellow poles are distracting, they're almost brighter than the car itself.
The wheels are dim, but this is probably because they're dirty. The lighting doesn't really make the car POP. This is hard to do at night when not really much of anything POPS because it's all dim lighting. If you're clever with photoshop, hue saturation, curves, and shadow/highlight you can salvage lighting fairly often.
I hope you don't mind I touched up 3 of my fav's from the photos.
Remember, photoshop is the digital shooters dark room, it's 50% of what goes into a good photo. GREAT photos never look the way they do straight out of the camera, there's almost ALWAYS post processing going on. So take your good and bad photos and just try and do what you can with them in photoshop, it's fun once you get a hang of all the controls. Use filters, use tools, use adjust and edit and highlights and burn and levels and curves, slide every slider to see what it does, they are ALL useful. It's fucking AMAZING how much you can do with Photoshop. EVERY time I use it I learn something.
Anyway here goes:
1. I loved this shot, but I just wanted to brighten up the front.

I used a single pass of "Shadow/Highlight" on almost their default settings. This DID introduce a lot of "gain" or the graininess to the shadow areas of the image. This is what happens when you try to revive shadows from an image, it's like jacking up the ISO on your camera, you're going to get noise. This is why it's best to do HDR multiple exposures. But for the sake of the image I think it came out just fine. The grain gives it a kind of, film look.This brought out a lot of the "dark" areas on the front bumper. But it also brought out a lot on the background.
So I took the "burn" tool, set it to burn "Shadows" at 10% and burned most all the background, giving it a more contrasted, dark look to draw the focus away from it.
I also did about 10% highlight reduce on the lowest range setting. This made the hood a little darker and brought out a little detail and color.
The headlights still weren't "bright and staring into my soul" like I like headlights to do at this angle, so I took the "dodge" tool, set it to highlights at 10% and ran it over the headlights a couple times. This made them pop more.
A final "curves" modification, one click on the center point of the curve, one point midway through the top 1/2 of the curve, and then one point on the bottom half. I pulled the bottom half down a little tiny bit (brings the "shadow" areas a little darker, giving more contrast, I like contrasty images, it's just my thing).
2. I like this image, it really shows off his wheels well. So I cropped it even tighter to make the subject even more "substantial", also it got rid of that pesky pipe and made for a much cleaner background.
Now his wheels are dirty, there's no helping that. But a white wheel should be WHITE. So what I did here is took my "curves" screen, clicked the "sample highlight eyedropper" and clicked on an area of the wheel. What his did is set the images very brightest point, in full white, to that pixel on the wheel. So what was once a light grey wheel, now is bright white. This also brought up the exposure of the rest of the image as well.
After this I did the crop to what I believed to be best. This is just matter of taste, always up the photographer.
I then did a shadow highlights on the image, low range, high amount, this brought out some of the lighting on the rear bumper which was mired in shadow.
What this also did though is made the rear window bright, and you could see all the dirt and grime. Again, can't be helped, Luis doesn't wash his damn car.
To do what I could, I grabbed the "burn tool" again, 10%, and did a couple passes over the rear windshield. This made the window much darker, hiding the impurities, but it left the stickers (highlights) unaffected.
I then did some hue saturation to bring out some color for effect.
Did a slight curve adjustment as before, and then cloned out the black mark on the bricks.
3. I probably went a little too far on this image, it looks "processed".

But I like it.
Step 1, I cloned out the pesky poles.
2, I used "quick mask mode" to mask off the car. This creates a selection area where the car is out of the selection so I can modify the background separately from the car. In quick mask mode you use the brush tool to "brush" your selection. What is selected will appear as red and when you exit the mode, it will become a selection area.
With the background selected, I reduced the saturation. The car is not colorful, the background should not over power it.
I then did a "levels" adjustment and brought the midtones way down, giving it that dark look, further reducing it's draw away from the subject of the image.
Now obviously my mask wasn't perfect, you can see this around the base of the side skirt and rear tire, where the asphault is still moderately exposed, this is because I did it fast, when you do it take care not to leave this areas because it makes the image look sloppy.
As before the front end was too dark, I wanted it to pop, I ran a shadow highlight pass to bring it out. I forget the settings.
I then used the dodge tool, set to highlights and 10% to really make those white wheels pop. Still, they are dirty and that can't be fixed, but they are now bright white.
I also dodged the turn signal a bit and burned the black areas in the bumper to hide some of the metal and stuff that got brought out in the shadow highlight pass.
Also burned the windshield to hide more of that signature "Luis Dirt".
And thats that.
My edits are by NO means good, they're just what I came up with. Remember photography is different to everyone so by no means should my techniques be the same as anyones. But I think there are some tips in there that you might be able to consider when you're next processing your images.
I recommend you also try shooting in the daytime for some of these as well. I remember the first 2 years I did car photography I ALWAYS did it at night because it looked "cooler", now I almost always try to shoot at dawn/daylight/dusk, the sun gives the colors of the scene MUCH more pop and it usually makes for a less brooding image. Plus night lights are always fluorescent or something and it makes for a really crappy white balance and harsh shadows.
And this has been "Photography Review" with your host, Skylar. It's 1am now, and I must sleep.
Keep it up though, you're improving at a ridiculous rate and I can't wait to see more!
EDIT: GREAT THREAD for post processing examples:
http://community.automotivephoto.net...read.php?t=584