A few people recommend them, especially those who score very well on the verbal. One member, for instance, tallied a score of 51V! He suggested Powerscore LR
From what I understand, these books show you a different way to approach GMAT problems. They also contain very difficult practice questions. These two books are tempting, but their price and large size make me think twice.
If you need a good RC guide, I would go with Powerscore's LSAT version. Manhattan RC
is not that great from what I hear.