Whether its Kaplan or Princeton or timbaktoo prep, never rely on any practise test score to tell you how good or bad you are doing. My max score on kaplan EVER was 590.

but i ended up with 710. Everyone's experience is different, but in general, i am sure everyone will agree that Kaplan Math questions are great for practise. I dont particularly like Kaplan RC. Kaplan SC/CR is ok, nothing great.
If you are feeling great about your math prep, try kaplan and see how you do. Dont worry at all if you dont do well. Make sure you analyze every practise test performance.
I got some time to kill, please allow me to ramble a bit.. it might help the new guys
IMO, Practise tests serve these purposes.
1. Improve your stamina/concentration
You have to take the test of 4 hours. the more practise you have and the more mentally prepared you are, the better.
2. Feel of the real test experience
Take full length practise tests.. that is.. write the essays completely. ( i used to skip essays, but i now feel that its better to have practise with essays). Make sure you take the two practise tests available on the Powerprep CD.
3. Timing (Only ETS)
If you want to check yourself on timing, the powerprep software is your best bet. I know the OG and Powerprep have questions in common. What I recommend is study the OG completely and then try the practise tests on Kaplan. there are lot of drills and short tests on the Kaplan CD that you can use to kinda warm up for the Kaplan tests.
As for the Marketing part, Marketing is full of tricks, some are genuine and then some. We just have to find out if the product works for us. Kaplan sure works , but dont take the scores very seriously. Prep Continuously and study your errors.
Hope this helps
Praetorian