Honestly, I felt like the 150 questions were more like the real test than UWSA (I haven't done any NBME so i can't compare that one); i read some forums where ppl thought the 150 free questions were more closer to the real thing than even NBME (that's what made me want to do them); and lastly, i read that 75 - 85% is considered a good score on the free 150 questions and anything above 85% means you'll do really well on the real test.
I actually spoke to one of my friends who recently failed the step 2 ck (183) and he said he had gotten a 67% on the free 150 questions
the usmle step 2 ck score estimator predicts:
with 80% you get 214 +/- 16
with 85% you get 231 +/- 16