I've been a customer of esurance since December 2001. The quote they gave me ($1090 per year) was $200 cheaper than everybody else, so I took the plunge. My experience so far has been good, though I had no accidents or tickets so far to test them (knock on wood!)
There are a few quirks you should be aware of. They claim to be a "totally electronic" company, so you will not receive anything by mail... instead, all the files, papers, insurance card, etc. are available in PDF format for you to print out. This can be both good and bad (I was able to print out my insurance card 3 hours after signing up, but now my insurance card is on regular paper instead of more sturdier paper)
I had to show my proof of insurance to the California DMV for registration, they accepted it without any problems...