I switched to Opera when I got tired of all of the Firefox memory leaks.
I did that too.
Originally, Firefox became my main browser. Then it really started to get annoying, and I tried out Opera and Chrome, and really liked Opera. I rolled with it for awhile, then started giving Chrome a little more love.
I have all four major browsers installed and up to date (IE, Firefox, Opera, Chrome), and use each from time to time. Some sites still work better with certain browsers versus others. Opera, at this point, has very little use on my system.
Opera still has some memory issues, but the main reason Chrome has taken over is because Chrome remains fast even when loaded with an unimaginable number of tabs, has extensions galore that help make it awesome (Firefox is better in this regard - but the memory utilization sucks!), and best of all: Chrome is basically THE best with web standards. IE is catching up with speed and web standards, but Opera... holy hell Opera is terrible about standards and website compatibility. So many sites are broken in Opera it's unreal.
Sadly, when it comes to government websites, Chrome sometimes works, sometimes refuses to play along, and that's basically the only time IE really becomes useful.