It's a decently high per-hour rate: $10-$14, depending. But for small round trips, on a semi-regular basis, it can be cost-effective over owning a whole car. I don't think there's a "catch" per se, just that it's a different pricing model (per hour, rather than per day) and a different distribution model: they outfitted the cars with coded key-card style locks, so you sign up, get a key, and when you reserve a car, they enable your key to open the car you're going to use. The big drawback to rental cars has always been that you have to go someplace central (like an airport!) to get one. I think they park these in parking lots.
/jordan