I have one of these:


It is similar in features to the Wagner mentioned above. While it is awesome for spraying stain on a cedar fence, or even latex inside a house, I wouldn't spray a car with it. If you are going to deal with the mess of spraying, you might as well use a real paint gun. Even a cheap paint gun is better.
The sprayer pictured above gives minimal control of the spray pattern... only pump pressure and paint viscosity. A real paint gun is more adjustable and will atomize the paint finer.

I haven't experimented with the roller process yet, but it appears much easier to master than spraying. A spray gun will allow you to make a huge mess in a hurry, in much the same way rotary polishers will allow you to do a lot of damage in a hurry. In the right/skilled hands, they are a quick way to get excellent results. In the hands of a beginner... maybe or maybe not.

If you do choose to spray, consider using a faster drying thinner to help avoid runs. Also, spray a few thin coats and allow them to get tacky before spraying the last glossy coat. Also seek out spraying techniques on some of the other forums that deal with more conventional paint and body work, like: http://www.autobody101.com/forums/