It looks useful....IF your app is already a success and has massive downloads/reviews. For those who only get a review every few days(guessing 95% of apps), how does your service provide value?
The idea is to help developers optimize their app's presence in the app store and get more downloads (hence get more reviews). We've had many indie developers use our product (even the free version) and increase downloads dramatically (3-4x).
This has been my experience. I 8x'd by downloads by using this site. They've got excellent analytic tools for determining which keywords are important in your market.
EDIT: Responding to grandparent, I only get a review every few days, and I have less than 200 reviews. Nevertheless, this report did provide some interesting insight. Too soon to tell if I'll be able to translate it into a valuable decision. But I like that sensortower has been innovating on ways to analyze your data.