I would disagree that a busy agent is necessarily a good agent. That is good for the agent but not necessarily for the client. A busy agent is a good prospector, which does not mean a thing to the client. There are many many busy agents out there that are not responsive and that don't provide the attention necessary. Also, I can look on the MLS at a lot of high profile, busy agents and see that their closed to expired listings ratio is very low compared to other agents.
Also, if I was trying to sell a house I would be wary of agents with lots of listings that are way higher priced than my house would list for, because you will not be a high priority for them.
That being said, I would interview several agents (busy and not busy) and listen to what they have to say, you should be able to tell if they have any intelligence or not and I would talk to some of their previous clients to see how responsive they were. A busy and responsive agent would be great, but I'd rather have a responsive newby with a good broker backing them up than a non-responsive busy agent any day.
Kevin