Smultron is your best bet if you want an open-source Macified text editor. (If you don't need the Cocoa experience, there are tons of great options.) Its strongest aspect is ease of use. Very simple, no distractions, et cetera.
It does however have some problems. Stability is not that great. Sometimes it'll beachball on launch for me. It does not, however, seem to crash after it is up and running.
Another issue is lack of features. I understand this is to be a simple editor, but it is only really friendly towards Algol-esque languages. If you want smart indentation for a language that doesn't use curly braces, you're out of luck. If you're primarily using Smultron for HTML and PHP (ugh), then you're fine.
It also has stiff competition from the free TextWrangler, which, while a bit crufty, is free, more featureful, and handles large files better. I personally use Smultron over TextWrangler nowadays, but it wins for me only by a small margin. To be honest, due to the languages I use (Scheme, Haskell, etc), most of my time is spent in smarter editors like vim or emacs.
In short, it works, but it has a long way to go. TextMate beats it if you can front the cash, although it is not perfect yet either (still some rough edges, and I've hit one nasty bug with it).