I took the practice tests for the real SAT and got 1500-1600s and got 1830 on the real thing. So I assmue they are about the same since the PSAT is very similar to the SAT. Meaning that, for me, the PR tests were more difficult.
My son took the Princeton Review PSAT prep last year. He was consistently making in the 220s on the practice tests and got 215 on the 2006 PSAT. He did forget a bubble on the math, though; so I would say they're pretty accurate predictors. Also, he increased from 191 Soph yr.
Apothesis, one of the things the PSAT does is to put things that sound correct in everyday usage as errors in the test to see if you know real grammar. "Would" usually does not belong in sentences with "if", it should be "were." That sounds weird to our ears since we usually hear "if" and "would" together, but it is technically improper grammar.