Hey Tom,
I agree with JM's reply except for the “paying clients” part. If you frequently develop UI scripts then it will save you time (which is money) and hair-pulling, head-scratching, and various other forms of self-destruction. 
I personally do quite a lot of UI-Scripting (usually unpaid), so for me it really is was a no-brainer when I (still hesitantly) plucked-down my money back in 2005 – it only took a few hours of use for me to realize what a good investment I'd made.
If you have extra bucks burning a hole in your pocket and like tools then by all means...
But do you need it? Probably not.
I've provided some pretty good tools for those who don't want to spend the money.
@CJK (somewhere on the forum) has provided a very nice script for getting a reference to the object under the mouse cursor. It's not as flexible as the way UI Browser does it, but it's still very useful.
Although I'm not very happy with Apple's incarnations of Accessibility Inspector, it is free and much better than nothing.
-Chris