I've been working with the Powerscore LSAT prep, and I think it is complete value. So far, I've been timing myself at 1:45 per CR question and 2 mins per question on RC. Here are the advantages I've seen:
1. Official test material. All official answers (unlike some 3rd party test prep) make complete sense.
2. Questions are generally longer, requiring complete focus. This is good not only to increase your reading speed but also to build your reading 'stamina'. I took my 1st GMATPrep about 6 weeks ago and the 2nd one 2 weeks ago. Verbal was up by 4 points, but more importantly I felt I was doing less guessing.
3. Contrary to what others may say, I believe the 1st 10 questions in CR make for good practice, even if you're not prepping for the killer questions. This is true even for RC, though the passages are not in increasing order of difficulty.
However, as already mentioned above, don't do any of this at the cost of OG and GMATPrep practice. The official material is the best, and most representative of the real test.
For SC, there is no better place to learn than the verbal forum here. In terms of ROI, studying SC gives you the best bang for the buck.
Hope this helps