Hi, guys, just wanted to second Eric in his post and elaborate a little bit as to why this is important (and this is not just a marketing pitch).

It is very difficult to write a test question that accurately mimics the official test - huge amounts of research, time, and money go into the process. I have written hundreds of questions - and it still takes me 1-2 hours just to write one question and its explanation. Two other test writers then have to vet it and then the question gets tested for accuracy, statistical validity, difficulty level, etc. It can take a month for one question to be cleared for usage with the general student population. It's critical to use sources that actually make the investment to get this process right.

It is easy to get messed up on topics or on question types if you study non-accurate representations of test questions. At best, a question just contains an outright error - I say "at best" because then we can see very clearly that it is a "bad" question and ignore it. A question can be technically correct, however, and have other problems; for example, it might not reflect the kind of material that appears on the test (I see this a lot with SC) or the kind of thought processes you need to use to answer questions. Imagine if you learned to play on the piano a song you'd never heard yourself - and your sheet music was inaccurate. You might learn the song perfectly as written (and maybe it would even sound fine)... but you'd get very poor marks from the judges if you were expected to play the song as originally written.

This is not to say that free sources (or even sources that don't have the R&D budgets of the big companies) will always write "bad" questions. I have seen good questions from free / less experienced sources. But there will be a higher incidence of bad questions mixed in - and unless there's an outright error, there's no way to tell the difference between good and bad questions unless you're already an expert. The more established companies only publish stuff that accurately reflects the test (we have to or we'd go out of business) - there are occasional errors, of course, but they tend to be typos and they are few and far between.

And, the disclaimer - of course, I work for one of the companies on the "recommended sources" list, so I absolutely have a vested interest in my opinion stated above. At the same time, the list Eric and I put together includes my company's fiercest competitors - so I'm not just saying, "Ignore everybody else! Just buy our stuff! Everybody else sucks!" Really.