I've always planned on doing it one day but never got around to it.
Water/Meth doesn't make more power in itself, but allows you to make more power by adding boost and/or timing by cooling the charge..... which you probably already know.
Can't hurt at all, however I'd probably be more inclined to put together my own kit than buying one of those pricey ebay kits. You can get away with using a much smaller water pump (like a window washer pump) if you do some tricky boost equalization on the bottle. Think I mentioned something about that in the "how to setup your fuel system" tech article.
Edit, yeah I did.....
http://www.rollaclub.com/board/topic/17418-how-to-set-up-your-fuel-system/