IMHO they're not really worth the money. The problems I've seen so far are not GMAT-ish, so to speak. Their style isn't like anything I've seen in GMAT questions and the scoring algorithm is way off. If you've exhausted any other source, then you could go for the 800score tests. If not, pretty much anything is better than those, except for Barron's.

I think there is more value in using the tests from the GMATPrep. Although there are just 2 tests, you can run them a couple of times and they pull out questions from quite a big bank of questions. So you should see new questions almost everytime.