I joined their beta program after using a couple of their 12,000 mAh iPhone battery chargers. Those chargers saved my bacon so many times over, from London to Paris, Australia to Africa - I never travel without one or three! If you don't have one, you should literally stop reading this and go get one. It is the one travel item that I literally triple and quadruple check to be sure I have with me on all trips.
That being said, the rest of their products have proved to be really solid and well thought out. I've definitely enjoyed being part of the beta program and have tested items from a pair of stylus for the iPad all the way up to very nice iPad keyboard cases. These aren't beta products either so far that I've experienced. All of these are already successful products, you just happen to be able to get to test out for free. I quite like being part of the feedback process that a company actually values and seeks out. How many times have you bought something and wished if they had only done this or that differently? With this pilot program, I actually get to provide input that goes back into the product directly.
It doesn't hurt that you get to keep all the items you test and continue to enjoy them. If you are wondering if this is something worthwhile to get involved with, wonder no more - you not only get great free products, you also get your voice heard and acted on, which is a rare enough thing for consumers nowadays.
You signed up already, right?

Good, now go enjoy your free stuff!