I agree with you insofar that I tried it and didn't quite get the appeal personally. But I had alot of experience with VIM coming in.
The degree of success is something I allude to as being congruent with the success of Apple products in general - well designed, cohesive, but definitely not the core demographic being power users. If anything you could probably say it was many's first powerful editor... and as people people grew in their abilities lament formed from its limitations, much of which was to be addressed in TM2, but the extended wait opened the door for people to move onto other, more powerful editors, and esp. set the stage for Sublime - which I'm neither here nor there with and may go back to VIM. But hey, its plugins are in Python and it has a VIM-like mode with an elegant UI so it's definitely a solid step in the right direction.
I think its success was well deserved given its core demographic and esp. tie-in with the Rails community which seemed to grow with it. I'm glad it existed, I consider it a great 'gateway' editor :) And now Sublime is its spiritual successor and I consider that a pretty good thing. Some people, esp. web designers, are just not going to give up UI polish no matter what so it is what it is. And it may very well have introduced others to VIM or EMACS who wouldn't have given them a look initially, finding the old school look and complexity daunting at first blush.