I am a developer who has stepped into the world of Rails very recently. I have learnt Rails 2.3.8 and have now landed into a major project in rails. The question we are facing is this: "Should we use Rails 3.0 for development or the older version?"
Our project has a time frame of 4 to 6 months for its release. Will Rails 3.0 be out of beta and supported by hosting service by then? Or should we stick to current stable versions of rails?
Rails 3 has hit the RC stage, so it should more or less be stable at this point. I'd expect that it'll go gold well within your timeframe. All other things being equal, I'd probably go with 3.0.
Consider staying with 2.x Rails. Not all gems support 3.x and those supporting are in beta. You should also use Passenger/Enterprise rails for more stability and optimizations (for 2.x Rails).
I don't think anyone can give a definite release date for Rails 3 even though its Release Candidate now. Regardless, I would go with Rails 3. It's far enough along that stability is not a real concern. Plus doing it now keeps you or somone else from having to do it later.
Rails 3.0 is working well. The problems I've run in to have been gems and plugins that haven't been updated for rails 3.
Check the gems/plugins you plan to use and see if they're compatible.
This is a hard question because @Zepplock is right and so is everyone else. It really depends on the gems you'll need. In addition, most of the stuff you'll find on the Internet to help you is also geared to Rails 2.x.
It's a hard problem, but I would say: if you can handle the possible problems of being an early adopter of Rails 3.0, it's worth it moving ahead. What nobody has mentioned are differences to Rails 3.0. It really adds some nice changes on all levels. Just the ability to use JQuery (like he rest of the world) instead of Prototype might make it worth it by itself.