I recently downloaded the new Opera 10. I have been using it for a few days now and I really like it. It starts up very quick, has great standards support and renders pages well, and has lots of great features. One I am really liking is the built in mail client. It integrates very well and is great for getting mail from my multiple accounts that I usually used webmail for. Firefox has been seeming kinda slow lately and I might switch to Opera as my main browser.
If you use Opera's M2 mail client with GMail IMAP you need to be aware of a few things because of GMails quirky IMAP implementation. Check these pages out to find out more.