We've been with Mercury for a long time. I periodically check, and haven't found anyone to beat the rates so far. Geico sometimes (including last year) purports to beat the rates with an initial estimate, but the final quote mysteriously rises by a couple hundred dollars by the time I get to the end. Really didn't understand it last time - I reported an old fender-bender for the initial estimate. In the final quote, nothing had changed except they eliminated the accident from consideration, yet the rate jumped 20%. Mercury ended up being cheaper after that.
I subsequently got the rate dropped quite a bit further when I started biking to work.