If you're hesitant to be an early adopter (AKA "guinea pig") Rails 2.3.x is very stable, and most of the useful plugins in the Rails ecosystem are still maintained for Rails 2.3.x.
Many big names are still on Rails 2.3.x, too. Forgive me if I go astray here, but GitHub and DocumentCloud are two examples that come to mind.
Staying on Rails 2.3.x is certainly fine for existing apps, but starting a new app on 2.3 is a very bad idea. You're setting yourself up for a lot of headache for the inevitable migration. Let alone depriving yourself of all the wonders of Rails 3.
That being said, nobody is forcing you to jump on a release candidate. Just use the latest released, stable version. In this case Rails 3.0.x.
Many big names are still on Rails 2.3.x, too. Forgive me if I go astray here, but GitHub and DocumentCloud are two examples that come to mind.