I agree with you gmat_perfect , that we must emphasize on official sources . There is lot of material available to practice that might not be in the official format , we have to be aware about the rules and must be able to identify . I guess there is no harm practicing extra bit until and unless it hamper or intervenes with the concepts of the problem solver . Lets see what the experts have to say on this .

I have a fear. The unofficial problems may not represent the actual test. Besides the unofficial problems may teach us some wrong rules by applying which we may be losers.

We can wait for some experts' comments.

Thanks.

I am actually going to write a blog post about this topic soon!

Hopefully my response doesn't make other experts not answer this one -- I tend to avoid threads where an expert from anywhere has already spoken. If you have favorite experts, consider sending them a message to ask them to reply.

Basically, my perspective comes down to two main points:

1. The OG should be the beginning of all of your studies, and you should come back to it later if you have time in your study program. This is its strength, and you can rely on its interpretation of style for the test.

2. The OG isn't enough practice for most people, and the OG doesn't see into future to new SC questions. Combined with the tangible benefits of regular CATs with questions you've not seen, I think other sources are necessary. If the OG was your starting point and you come back to it regularly for review, you are less likely to fall victim to something strange in another source. I think it's fine for non-OG sources to test other aspects of grammar (rather than style) or test the same topics as the OG in non-OG ways, because you don't know what future GMATs will hold, and it's not as though being better at English will hurt once you're in your MBA program of choice -- but again, this is based on keeping the OG in the background at all times.