It's going to depend really. When I moved e-pubs (under my real name) I contacted a few authors at the new one, and not all were 100% happy. But I could then decide whether I'd be happy or not with what they were describing. What is great for one person (or something they're prepared to put up with because of other benefits) might be a dealbreaker for another.
People stay with agents/pubs for all sorts of reasons, and not all of them because life with them is super-smashing-great, or they are the world's best agent. They don't necessarily leave an agent because the agent is bad.
This is a great starting point to think about what you, personally, require from an agent. I'm quite happy with me and my agent only talking when we have something to talk about. I know another writer who told me he couldn't live with an agent who wasn't able to talk him down off a ledge, and others who get the jitters if they don't get an email a week. I have never spoken to my agent, or met him, and other writers sometimes look at me aghast when I say that, but it works for me, and I'm happy with it. It might make you have a conniption.
So my agent is great for me -- but might well not be a good fit for someone else. Not because he's bad, but because of the way he works, and different writer's needs.
Obviously this is past your initial 'Well, have they made any sales in the genre I write in?' 'Is he a scammer/vanity press front?' etc business side of things.