So - anyone try Fedore Core 3 yet? Is it any good?

I have it on a box at home. Its pretty smooth. It works very well, extremely stable. Haven't run into any problems at all so far (well, its only been a day ;) )

The most impressive thing about it for me was it can automatically setup LVM for you during the installation. Its an exellent OS, I would highly recomend it.

Is it worth upgrading to or should I stick with FC2?

Just installed it myself as well on a spare box at home. Don't see any major issues at this point. I'd recommend the update, but also encourage you to read more about it on your own before doing so.