I haven't taken the GMAT yet, so take this with a grain of salt.
My experience with Kaplan tests is... up and down, so to speak. When I took the free online test, my score was 690 (q46, v41). This came as a shock: I had only three mistakes in the quant section, all miles apart. This is why I was extremely frustrated with their scoring algorithm. But I was stubborn enough to take the other 4 tests that come with the Premier program, and I was amazed to get some off-the-charts scores: 53 (again, only 3 mistakes) and 55 in quant, and even a 55 in verbal.
This is why I believe that the opinions of many are well founded: the free test is designed to give you a lower score so you'll be tempted to enroll in their course, while the others are decently graded.
HOWEVER
As I've said before, the maths in Kaplan is slightly different form what you'll see on the GMATprep software. Kaplan maths seems to be (and I quote from their Kaplan 800) "straightforward" or more academic. It's not hard if you're a recent college grad, but it is challenging if your skills are a bit rusty.
On the other hand, official questions tend to catch you off-guard because they are confusingly worded. If you don't read the questions very carefully or are not able to translate words into equations and so on, you're in for some trouble.
IMHO, Kaplan maths is not more difficult than GMATprep maths; it's just different.
The same goes for the verbal part: nothing can really replicate GMAT questions 100%, but they try, so to speak. I personally found the verbal part to be more difficult in Kaplan than in GMATprep. This is particularly obvious when I think of RC: in both GMATprep tests, I had zero mistakes in RC. The Kaplan passages seem to be a lot longer and I missed between 2 and 5 questions.
The CR in Kaplan is... I don't really know what to say about it. It's just very confusing! I had a few instances when I could have sworn that I picked the right answer and their explanations just didn't make sense. The only tool that came close to the real CR is the Powerscore Bible. It's a shame it has only very few practice questions....
All in all, don't get disheartened. I say try to practice with the official stuff now, that you're almost there.
