I work for one of the major insurers named in this thread. I do not work for USAA, but I would say that if you qualify for coverage with them, they are an excellent company with stellar customer service. They regularly beat us on customer satisfaction (but we are very high on the list). Their prices are generally competetive because they insure a group that is low risk. I would switch to them in an instant if I qualified.
Other than that, I would say to go with one of the big guys, and not just because I work for them. I was a claims adjuster for a while, and I will tell you that it was almost impossible to get anything done with one of the smaller, fly-by-night insurance companies. Their agents are overwhelmed so it is hard to get them to return a call, their contracts allow them to disclaim coverage for what I think are totally ridiculous reasons, and generally their customer service leaves you wanting.
What you want out of your insurance company is really up to you. Some people want to deal with an agent for everything, but you will pay more for that (generally). I, like Mr. Macinstache, want to do everyone online and by myself and I don't want to talk to a human, so I prefer the direct marketers (GEICO, progressive, etc.) with robust websites. They tend to cost less as well because their overhead is much lower.