GMATPrep is the closet one in term of actual GMAT simulation and the algorithm used to calculate the score. Regarding tests simulations provided by other vendors, we need do a comparative evaluation of the score obtained. Take any test and compare your score to that of others. Go to "I just beat the GMAT" and compare your score with that of others who have already beaten the GMAT. Check their final score. That will give you idea, how you are doing?

Here are my perception about few vendors:
5 tests in Kaplan CD: Anything above 600 is good. Verbal is too tough and doesn't exactly simulate GMAT style. Your final GMAT score could be something 60-80 plus your Kaplan score.

Princeton: Read it's easy, but the final score would be somewhere near to your Princeton score.

MGMAT: Quants too tough, but the best to practice for the real GMAT. Scores will be close to your final score, though sectional score might vary a lot.

Thanks for your help. I have tried a book of Kaplan (Which I have borrowed from the library). I only got 73% accuracy in Quantitative part. Verbal part is even worse: I can finish only 50% within the time limit and only get 70% correct.

I agree with your point that Verbal part is not reasonable. I don't think anyone can complete a verbal part with 4 Reading topics.

I would like to ask one more question about the essay part. I recently use the scoring system of findscore.com (https://findscore.com/). It gives the analysis of the essay after the submission. However, it seems that many people got a score of 5 easily. Does the system of this web site evaluate the essays accurately? I am so frustrated about the score it gives me.

If you have got a better essay evaluation site please let me know. Many thanks for your help.

Recently I took the GMAT and scored 5 in AWA. I haven't used any site to evaluate my essay. I just followed one book: "GMAT CAT Answers to the Real Essay Questions." It's a very good book to develop ideas on topics that comes in AWA section, dos and don'ts, and what matters in AWA. The book first tells strategies in detail and then provides sample issues and arguments on topics listed in OG11. However, finally you have to develop your own strategy. Before test, you should have your own template ready.

If you want, I can share my AWA startegy and template with you.