If your goal is to prepare for the test then Kaplan is great preparation.
If your goal is to get an indication of your actual scores then use Princeton Review.
Don't get caught up in predicting your score. From my experience and from reading a lot of posters, they all complain about the lower Kaplan scores in comparison to the other products, but remember Kaplan "guarantees" a higher score.
I scored 120 higher on the actual than on Kaplan and I don't think it is that unusual. I read in another post that the average increase varies from 80-100.
Again, if you want to be prepared use the Kaplan. If you want an idea of where you will score, the free Princeton Review test online was within 20 for me.