Opera as a browser and email client is very good indeed. I paid for it (a while ago) and don't regret it. The only thing it lacks is a scheduler.
Interesting that they're now releasing Opera for free - as a combo it's pretty unbeatable for speed and security.
I use Firefox almost exclusively because what with Thunderbird (the email client) and Sunbird (the scheduler - still in beta) it just works for me. And I can X-platform with the Linux boxen.
Gotta admit that Outlook (NOT Outlook Express) is very polished - pity about the security worries (and Microsoft's persistently unpleasant behaviour).