I've been using Feedback Express for over a month and saw an immediate doubling of feedback received. Very easy to use and setup, with excellent support from the customer service team. Plenty of additional features for more customised e-mail campaigns that I haven't even got round to using.
Probably not too useful for smaller sellers, but if you receive more than a handful of fb comments a day it's an easy way to increase this significantly and keep in touch with buyers.
Overall if you're looking to increase feedback or mass-mail customers, it's a user friendly, feature rich tool that does a simple job with little fuss.