I see that many people here are working out of the 1000 SC book to improve their sentence correction skills. Is this book worth getting...are the questions tough?

Also, are the questions derived from real GMAT tests, or have they been created by other experts (i.e., non-GMAT authorities)?

I noticed that someone on this forum warned people not to work out of 1000 SC in 2006. Is this because the questions are somehow defective?

Finally, how can this book be obtained...is it only downloaded via the Internet?

Thanks!