Certainly you can trust the GMAT prep cat's...It happens on the real days sometimes things changes due to anxiety .

Also its CAT exam if you get easy ques wrong dues to silly mistake your scores will drop in big margins....

So you must watch out for those tiny easy ques as well..

I agree with everything Prateek said, but I'd also add this:

In asking "Can I really trust the GMAT CATs?" you're asking the wrong question.

Practice tests can be decent indicators of how well you COULD score but that's probably the thirtieth most useful thing that practice tests are for. So among those who primarily use practice tests as indicators of how they WILL score you'll probably find a disproportionate number of disappointed examinees. Those people aren't using practice tests effectively.

Practice tests are fantastic indicators of **why** you'll score what you'll eventually score. They tell you which mistakes you tend to make when you're under timed pressure, which question types tend to throw you off when you're doing an entire test and not just a Data Sufficiency specific session, etc. They tell you where you're prone to wasting time and they show you how difficult it can be to stay focused for the entire exam. They help you prepare a worst-case strategy (how far behind can you fall on pacing before you have to guess to get back on track?) and develop a sense for when to cut your losses on a problem and when you should take the extra time to get it right.

Practice tests are about the experience much more so than they are about the score. Think of it as a scrimmage or a "friendly" soccer game - if Spain loses a friendly to Argentina but learns about how its youngest midfielders are vulnerable to overattacking and sees some opportunities to better deploy its substitutions, that's a plus for the next World Cup. But if it pulls out all the stops, plays all the starters, wins the match but doesn't learn anything that can help it in 2014 is that really beneficial?

Practice test scores are fun to watch as they rise - they can help you build confidence and see progress and those mental cues are important. But practice tests are much more useful in their diagnostic potential, so don't overlook that. The only score that matters is the official exam score, so don't fall so in love with practice test scores that you neglect the lessons from those tests.

Thanks prateek_guy2004 and Brian for the your responses.

As a Brazilian, I really liked your soccer game example. Nice advice and I will take it into consideration on my next cats.

I just hoped that Veritas would sell me just the 15 cats....