Given the relationship that coders often have with their editors, this is like saying to some parents "It's just a boy"… If you spend a lot of time in one program voluntarily, it's hard not to develop some feelings for it, beyond pure utilitarian reason.
TextMate is certainly one of those programs. It embraced the programmability of a Unix-based environment, coupled with a very simple design and actually introduced some new concepts (or at least made existing ones more prominent and/or easier to use). It also came at the right time, filling a niche for developers that wasn't satisfied by the pure-Unix breed (vim, emacs), nor by the entrenched MacOS editors (well, BBEdit mainly). Being introduced in a quite hip Ruby on Rails screencast didn't hurt, either.
Don't get me wrong, I'm not even a hardcore TM fanboy, as I switch a lot between editors, while secretly regretting that I can't just stick to emacs…
But I certainly understand the appeal, and if a bit of money (that most developers who buy Macs certainly can afford) helps keeping the momentum of a resurrected application, I'm all for it. Whether I'm buying it, is another matter. Right now, it doesn't fill an interesting niche for me, and I'd much rather spend the time customizing my .emacs file. Never mind that where I'm really missing good editing functionality isn't HTML or Ruby, it's Objective-C (or Java, when that was still job-relevant), where an IDE really helps. Well, let's see what AppCode[1] can bring to that particular table…
> "I've never used it; is it really /just/ a text editor?"
It's not a text editor in the scheme of Notepad or TextEdit; it's an editor for text. That is, it's a tool for programmers, with all the searching, syntax highlighting, and whatnot that goes alongside. Sometimes even writers use it.
What makes TextMate good is that it executes well, and doesn't (at least to me) try to be an IDE. Which is why many of us like it, or at least liked it, before things stagnated.
Edit: to clarify, my use of "just" wasn't meant to be dismissive or diminutive in any way...