I am a non-native speaker, although I have lived here for the last 14 years. I took the GMAT less than a week ago and I got 39. While this is not a stellar score, I am happy with it.
First off, I think GMATPrep is exactly like Pearson's GMAT. I was always averaging 39 - 41 on the GMATPrep and 39 is exactly the score I got.
The questions didn't seem any more difficult than the ones I found in
the Official Guide. My friend teaches the GMAT for the Princeton Review and he gave me just two tips to use on the SC:
1) Look for grammatical errors (idioms, modifiers, etc)
2) pick the most concise one that does not change the meaning
I do agree that CR questions are pretty difficult, but they are also difficult in the GMATPrep and
the Official Guide... Sometimes the "assumption" questions lift things straight from the reading portion (which, in my opinion, is no longer an "assumption" and a "reasoning" or a "fact")... basically CR has it's own language... but
MGMAT is very helpful with this.
For me, I found the RC portion to be the section to really boost my Verbal score, then the SC portion, then CR.
Overall, I found
the Official Guide and the GMATPrep to accurately reflect the types of questions you will get on the GMAT.