Everything I've read and heard about water & water/meth injection is the same Harry:
"Stay away from it."
It was a technology invented for old aircraft engines and adopted by car modifiers as a cheap and easy way to get more power out of their engines....
It's a band-aid fix for detonation problems that could be solved through a good tune, good setup and good fuel and shouldn't be required if all those basic requirements are met. I very much doubt you'll need to use a setup like this on your car and if you did, I'd be very very cautious about relying on a system that could easily let you down and result in costly damage.
As always - your choice at the end of the day mate, but I reckon your $$$ could be better spent on other (and more beneficial things) than something like this.
PS - if you look at Dan's last few posts and see how he's almost running 11s without water injection... it begs the question as to WHY you'd want to run water injection!