There's a lot of excitement about Firefox 2.0 (although it seems muted compared with 1.5) but I have no plans to jump on the bandwagon quite yet despite the new version's better security and better speed. Why not? It mostly has to do with the compatibility of extensions. Why would I want to jump to 2.0 if some of my 1.5 extensions won't work? It would be like buying a new car with a better motor but the headlights and speedometer don't work. For now, I'll stick with 1.5. Who knows, I may even try IE7.
Update: Hardware 2.0's Adrian Kingley-Hughes has a post looking at 2.0's shortcomings, including memory leaks, weak anti-phishing features and incompatible extensions. He still believes 1.5 users should upgrade: "If you stick with 1.5.x then you're going to be at risk from unpatched vulnerabilities.  The only way to go is up to 2.0 and keep downloading updates as they become available."