Hi hradsky,
While working through individual questions and quizzes is a standard part of the GMAT training process, your results from that type of practice won't necessarily define how well you will perform when taking a FULL-LENGTH CAT/mock under realistic, test-like conditions. That's one of the reasons why you have to take those CATs at regular intervals (and again - under realistic conditions) - to measure how well you will handle the FULL Exam.
While the most realistic CATs available are the 6 from GMAC, the CATs from Kaplan,
MGMAT and Veritas are all 'close enough' to the real thing that they will provide you with a relatively realistic score assessment (assuming that you correctly take the CAT in a realistic fashion). As such, you shouldn't be concerned about 'burning' a CAT - there are plenty of high-quality CATs that you can use during your studies.
Before I can offer you any additional advice for your studies, it would help if you could provide a bit more information on how you've been studying and your goals:
Studies:
1) How long have you studied?
2) What study materials have you used so far?
3) How have you scored on EACH of your CATs/mocks (including the Quant and Verbal Scaled Scores for EACH)?
Goals:
4) What is your goal score?
5) When are you planning to take the GMAT?
6) When are you planning to apply to Business School?
7) What Schools are you planning to apply to?
GMAT assassins aren't born, they're made,
Rich