I've been reading e-mail with Netscape since 1996 or so. I jumped over to Mozilla when it was at .99-something, and never looked back. Mozilla got noticably more stable as it went from 1.1 to 1.2 and so forth. The new Bayesian anti-spam features are particularly noteworthy.

Of course, Mozilla still has its problems, but I'm sticking with it until something radically better gets stable enough to use. I'm particularly intrigued by Haystack, but it's still early in its development.