I am using 800score, KP and PR. First, I am not a high scorer, so my comment may not count for anything. Also I think the usefulness of those studying aids depends upon your expectation of your score and what specific areas you want to improve.
I think 800score is good in math practice and I dont have a 'good feel' on its verbals. Their verbal Qs do not have the 'same feel' as the real Gmat.
The kp is good overall but their RC & CR, IMHO, are more diffcult than the real ones, though the questions' style is similiar. I think SC Qs are about the same as the real gmat.
The PR's math i think is easier than the real gmat, but their Vs' styles or diffcult level I think are very close to the real test.
just my 2 cents!
*btw, currently i m scoring mid600s on PR and 800score and low600 on KP.
I have 2 months to go and am hoping to score a 690 - a median score for my dream school's Part time program.. i m still far from the objective. *need some prayers