If nothing else, UWSA is good because it gives you another 200 high-quality UW-style questions. And it was only another $10 to add on to my UW subscription.
The scoring curve is friendlier than the real thing, but the questions are more difficult. With the NBMEs, the scoring curve is harsher than the real thing and the questions are easier. You can expect your real exam to be somewhere between the two.