I didn't use it that much, so I can't say if it helped a lot. But I will say that on my actual MCAT, there was a free standing question that was basically the exact same question with the same answer choices as one of the KA soc questions. Of course I didn't see it on KA until after my test so I guessed and got it wrong!!!
That's just one question though. But it was very specific and not mentioned in the Kaplan or TPR books -I checked. However, it was discussed in the free soc wiki that the AAMC links to. They have a huge psych/soc reference table with different book/web resources to check out on their website that covers the section's topics. (It's a lot and I never did check them out before my test, still managed a 128 on the section though.) Anyways I would use KA for at least some practice questions.
My only preparation for the section was the Kaplan book and practice passages from: TPR full lengths (& science workbook), Kaplan full lengths, aamc section bank/full lengths, and KA passages. If I didn't feel like taking a 7 hour full length from Kaplan or TPR, I would just practice individual sections when needed. So that's also an option for more practice material.