Water on the Glass is neat animated wallpaper, but it asks too much of you once you try to install it. You'll have to give up your privacy and accept a nasty ad agreement in order to gain access to this wallpaper. It's tough to imagine most people will be willing to do that when there are other rain-themed backgrounds available.
This app isn't listed on the Google Play store, so you'll have to find an alternative means of downloading it. No matter how you get it on your phone, Water on the Glass -- which appears as "Rain on the Glass" in your phone's menu -- asks for all sorts of permissions when you install it. It wants full network access, GPS access, and even your phone's call state and other personal data. Since the wallpaper doesn't geo-locate you to show you if it's actually raining where you are, it shouldn't need access to all of those private details. When you first select the app to make it your wallpaper, it tries to make you agree to an ad agreement that will put ads on your browser and search. Like most live wallpapers, this app will drain your battery and put extra strain on your processor if you're not careful.
That should be the least of your concerns considering all this app needs to work, though. Water on the Glass doesn't offer enough to make up for asking you to accept the privacy risk it poses. Most people will be more than happy with the similar wallpapers you can find in the Google Play store.