PairofSox- I used to be an assistant at abrams in the commercial department. I don't really want to post all the plus/minus stuff about them, but if you have any questions, you can email me privately.
Berrebbi- I think there are a bunch of factors that determine what makes an agency good or bad for each individual. It's hard to say that one agency is better than another. The most important things with commercial agencies are 1)Relationship between agent and actor, and 2)relationship between agent and casting director. If you are with an agent, ask them which casting peeps they work with. Beth Melsky is a BIG one (even though she is...not so nice). It's good to have an agent that gets along with her. Agents will always pay a helluva lot more attention to signed clients, so that's another thing to consider. I guess this is sort of general, but I just think each experience is diffrent.