I painted the exteriors of both the
Boler and the PlayPac (photos in the link in my sig). With the
Boler, I used a roller. For the PlayPac I went with rattle-cans (not as pricy as you might think). Both were MUCH cheaper than taking it in for a pro job, but the quality would have been less as well. To really guage that you'd have to compare different paint jobs in person though. Photos aren't gonna do a good enough job.
In brief: The rattle cans gives you a much smoother finish. This can be REALLY enhanced by giving the whole thing a high gloss top coat (didn't do that with the trailers - but I DID do it with a set of
fiberglass saddlebags I have on my bike). The roller leaves a bit of a texture behind. The rolled on paint was much thicker as well. Upside of this was that the rolled paint does a better job of covering any imperfections in the body of the trailer than the spray will.
I haven't tried it - but I've heard some report very good results (ultra-ultra smooth and glossy) with polyurethane paints (for glass boats).
I'm a real sucker for the two-tone look myself (which is obvious if you look at the photos).
Mike