I just started a Rails project a few weeks ago with Rails 3, and I've been quite happy so far.
Some gems/plugins don't quite work yet. For example, Selenium and friends seem to be a little behind, though after trying a few plugins I finally got it working fine through Capybara. In_place_editing isn't working out-of-the-box for me (I suspect because of Rails 3), though there are alternatives and it's not complex at all. And I had some trouble with factory_girl, though apparently there is a version for Rails 3 now.
But in general, most plugins I've tried seem to be working fine at this point. Maarons has already pointed you at RailsPlugins.org if you want to check for a specific plugin.
Finally, there are bunch of things that are just better in Rails 3 (see the release notes). I used Rails 2 for a smaller project a while back, and using Rails 3 now, I was pleasantly surprised by the new routing (much less confusing), and the added bundler support (makes deploying much less scary).
Since you're setting up a new project, I'll also mention that I've been quite happy with Ruby 1.9.2 (as opposed to 1.8.7). Check PragDave's blog post for some major changes. The only thing that I recall needing explicit tinkering was the debugger -- just use the ruby-debug19
gem and you'll be fine.