I've been using the recently released Firefox 3 Beta 3 for a few days on my home machine. It's not quite ready for prime-time - I've found that in particular a lot of AJAX stuff, like on Twitter, doesn't work. Not sure if the implementation is just not finished, but that's a little concerning as a developer.
There are two extensions I can't live without - del.icio.us and FireBug - and sure enough both are "incompatible". Luckily, it turns out you can make them load by turning off the compatibility check. I've been using them for a few days now with no crashes.
Even though it hasn't been tuned yet, it definitely feels faster and lighter weight than Firefox 2 (which as we speak is using well over 400MB of RAM with 13 tabs open).
If you want to try it out, I would recommend installing Firefox 3 side-by-side.