I use Rubymine for most of my Ruby/Rails/Sinatra development on Ubuntu, and use it in conjunction with TextMate on OS X. I find it convenient enough to alternate between TextMate when I don't need IDE features, and Rubymine when I do.

One of the biggest improvements is that indexing now occurs in the background and auto-complete and other features become available that depend on knowledge of an application and the gems that it uses.

This is subjective, but once Rubymine 2.0 loads up and is done with any background indexing then the CPU use is minimal, and I think improved from earlier versions (nice to not have the fan kick in on my laptop when the CPU cores heat up). For the Rails application that I am coding on right now, Rubymine is using about 360MB of resident memory - this is OK with me.