Sublime is nice, and would probably be the best choice if you were choosing your first text editor. If you are coming from TextMate, it depends on your situation. Sublime is faster, it has a nice always-on, miniaturized preview of your whole document, and it has tremendous momentum in the developer community. I'm primarily a designer who writes JS and CSS, and, so far, Sublime's benefits don't outweigh the arduousness of committing different keyboard shortcuts to muscle memory. So, I have both installed, but I still use TM primarily. My good friend, who is a much better programmer than I, prefers Sublime. He is a fast learner and so his "switching cost" for new editors is relatively low.

Sublime Text is very similar to TextMate, supporting many of the same keybindings, snippets, and themes. (I've even set up my system to use Ryan Bates's Railscast theme to keep things looking familiar.) Unlike TextMate, Sublime Text supports multiple panes and is cross-platform.

Because of its advantages (being cross-platform is the clincher), I'm planning to use Sublime Text in place of TextMate in the 2nd edition of the Rails Tutorial screencasts. I also hope to make a Sublime-specific screencast at some point, with an emphasis on keyboard shortcuts useful for Rails development. Further updates as events warrant.

Much better than TextMate. Faster, actually actively developed. Just cmd-t and cmd-p makes it worth the switch.

Sublime blows TextMate out of the water. Hands down.