I've heard all of the arguments for water powered back-up systems, but I have even more arguments against them. My biggest complaint is that there is no way for these systems to do a weekly self-test, which means that this can literally sit for years before actually being needed. When that time comes, there's no guarantee that the pump will actually work. I have seen this happen multiple times with the end result being that the basement flooded. These water powered systems have no alarm either, so there is no way to know when they have been activated. These systems connect into the domestic water system for your home which means that they are now required to have a back flow preventer installed to prevent cross contamination. Here's the best news, back flow preventers MUST be tested annually to ensure proper operation. This test itself can cost anywhere from $75.00 to $175.00 and the water company can shut your water off if you don't have this back flow tested.
The best advice I can give you is to not install a water powered back-up system. Spend the money to have a quality battery back-up system installed by a licensed plumber.