I bought the S41S version so its the one I have more experience with. Like Tachead said there's plenty of reviews already on all the major forums, but very quickly I think its a great little light. The host is very nicely made and, for the most part, the electronics are solid. I wish it came with the tiny25 driver instead of the '13, but that's an easy fix if you're so inclined. Also it needs 219c's for their more favorable Vf with this small cell, but again that can be fixed.
But don't let me dissuade you buy saying that I ripped mine completely apart and rebuilt it. Its nice as it comes and easily "tweaked" if you want.
One word of warning: there were some issues with the first runs. I believe all lights currently shipping have been upgraded but if you're buying a used one beware there could be issues.