I think it all depends...

I have had 3 thus far. Right now, I am not using an agent.

Agent #1 - Once I signed, I never heard from him. He only seemed to want to collect money off of me - never returned letters, or phone calls.  When I didn't hear from him (he was to send me monthly reports), I sent him a letter. When these went unanswered (even though he continued to send monthly bills for phone calls, postage, etc) I called him. I called three times; once per week for three weeks and got the answering machine each time. When he never called back, I canceled my contract with him. Thankfully I sent it registered mail because he never answered that either. -- I definitely got taken on this one!

Agent #2 - She was an agent from the same agency (but...). She said she had seen my letter, and had learned of my unanswered phone calls. She was going to start her own agency and since I had canceled with him, she wanted to represent me.  -- I used her for a short time, but then due to personal issues (divorce & supporting 3 children on my own ) I put everything on the wayside. We didn't have a formal contract as she was doing it on spec, so to speak. So, we ended the relationship.

Agent #3 - (Years later) A very close relationship. She not only answered my questions, we constantly traded phone calls, etc., and she was great on sending me reports and comments from publishers. Unfortunately, but happily for her, she got married and as her husband is in the military and they move from place to place, she closed her agency. -- she taught me a lot, and I keep in close contact with her as we have become friends. I am going on my own now (without an agent), but with the knowledge and friendship I have with someone who knows the ropes. 

I realize that some houses only accept agented submissions, but I will cross that bridge when I come to it and probably think about another agent at that time.