I used them for a paint job 12-24 months ago, and I was pleased with the work. It was 50% of what a local body shop gave me for the same work.
I was told to look for a "Maaco certified" shop (not all Maaco shops are certified), which is a shop with a higher level of performance. I'd look for one in my area and go to them if I were you.
Along with the paint job, they did a few dents for a very reasonable price($15 or so for each dent). That work they did was excellent.
As far as having a bad reputation for low quality work, I would suspect most of their bad reputation is because they offer a $300 paint job with a 6 or 12 month warranty. That is no clear coat, no prep, so people try to save money and then wonder why it looks bad after a few months. I did a 5 year warranty separate clear and base coat that cost $1500, and 18 months later the car (my civic) still looks very very good. So just be sure to tell them to do any and all prep that is involved in working on your bumper (if you don't specify it, it probably won't be done). I don't know what prep work would need to be done in your case, but as long as you had that approach, you should be fine.