Water/methanol injection does make a big difference for boosted applications - you can run more boost without detonation, which is important when you are starting out with a relatively high compression engine and poor quality california fuel. The most common systems are aquamist and the jackson racing system, which is actually an aquamist as well. I have an aquamist which I'll put in my turbo RSX if I ever finish it. The snow performance system looks like good value for money, since it comes with some parts which the aquamist does not (a water reservoir for one). The pumps look different between the snow performance and aquamist - the snow pump looks like a fuel pump, whereas the aquamist makes their own magnetic coupled pump.
For NA there is no advantage running water/methanol injection. The water vapour displaces air and reduces engine output. There have been a few studies where people have measured BSFC vs power using different water/air ratios, and the conclusion is that you lose a little power with water injection but can run quite a lot leaner and obtain better BSFC. Perhaps this is relavant today with the price of fuel.
The weak point of the water injection is the pump and nozzles. Some pumps are unhappy running the whole time (the aquamist states a 1 minute duty cycle) and the nozzles are prone to clogging. Using distilled water and filters is a good idea.
Methonol is nasty. It will eventually corrode any steel it touches and eat some rubber and plastic. You much be careful not to allow any to remain in the combustion chamber once the engine is shut off otherwise the rings or spark plugs will rust in a couple of days. However mixed with water for water/methanol injection you should not have any of these problems as long as the pump and nozzles can handle methanol.
