I've been using the very popular Planetbike Superflash for a
few years with no problems. Many times in heavy downpours:

https://www.youtube.com/watch?v=W7jf...IoDLA&index=47

I switched when Cygolite came with the brighter and USB chargable Hotshot.
Had it for about 2 years now and no problems so far even in steady/heavy rain.
I use fenders though on all of my bikes that I take on rainy rides:
BROOKS C17 LOANER FROM BFOLD by 1nterceptor, on Flickr