I think the general concensus (and I agree) is that the GMATPrep questions are tougher than the real thing, but just slightly. However, the scoring is pretty accurate. I hear people mention their ire about the scoring of the GAMTPrep, but I think it is accurate IMO.
Now if asking if the GMATPrep will be an accurate indicator of your real score, then I would say maybe. IMO, if you haven't touched the OG, then I think the PowerPrep is a great indicator of your real score, but if you have read the OG, then the GMATPrep is better (even though there are a couple OG problems in the GMATPrep). Again, this is just my opinion.
Here is my breakdown:
PowerPrep 1: 730 (before OG)
PowerPrep 2: 710 (before OG)
read OG
GMATPrep 1: 730
GMATPrep 2: 740
Real thing: 710
I hope this helps.