for the price (is it 19.95? 29.95?) it's not bad, plus the tests are very portable and accessible from the Internet if you are planning on studying/practicing at work or something.
The preparation thing they have is good because it is concise and to the point, but you shouldn't rely just on that alone.
The tests themselves... I don't think they are very close to the real thing, but it's good practice and they have a built in test pacer (it tells you what question you should be working on) which is very helpful (if you follow it).
The scaled score calculator is not very accurate.. from what I have seen, about 20 points difference (ETS was more generous) in my case.
One bad thing about the tests is that they have a pause button. Although this is convenient, it is a luxury you should really avoid using. Even though they have a little message that says "this feature is not available on Test day"...
Anyway, I bought it, found it useful... but should not be used just by itself. I would say get as many resources as you can afford. After all, you are investing in an MBA. :D