In my opinion, it would be a lot more productive to learn one of the "standard text editors," namely vi(m) or emacs. The learning curve is higher, but it will allow to be very productive on almost any Unix-based operating system.

Textmate is a great product with a lot of useful features right out of the box, and it very Mac-like with its UI and Apple-style keyboard shortcuts. It also has a lot of macros, and you can access a lot of shell commands straight from the menus. However, by hiding all of these details, it makes it more difficult to use a computer that does not have Textmate on it.

Thus, if the "Macness" of a program is very important to you, and you don't mind being dependent on one machine and one proprietary program, then Textmate is certainly worth the cost. If not, then I recommend you save the money and build skills for the future by learning how to work with vim or emacs.