Subethaedit was my favorite OS X text editor (although I regularly use vim). I never used the collaborative feature, I just used SEE as a standard GUI editor, but when it became payware, I had to decide if it was really worth it.

I decided to pay for an OS X text editor, even though I am happy with vim. However, the feature sets I found worth paying for were in TextMate, so I bought that one instead. You should take a look at that before making a final decision.

Older versions of SEE are also available for free for non-commerical users.