I used them in CUN in February. They were so dirt cheap: <$10 per day. I booked through Orbitz I think.
When I got there I had to ask around for where their shuttle was, but wasn't a big deal.
When I got to check-in counter, they made me pay $11/day for insurance, saying that it was mandatory. I thought it was a scam, but they absolutely insisted saying that it was against Mexican law. Since this was only a two-day rental, and it was still less then $50 all-in, I let it slide.
No idea if this is a Mexico thing or what.
Anyways, the car was in acceptable condition - no issues. Returning the car took almost 30 minutes, but was fine.
Basically I got what I paid for.