With an energy conversion efficiency of usually 75 to 80 % they are really efficient and don't have as much energy loss as other types of energy storage. It's a simple, but powerful concept and I find it beautiful. However, there is some concern regarding their impact on the local ecosystem. Not only do they need huge water reservoirs, which are artificially created and therefore might impact nearby rivers and even fish migration, but the way they are sealed with concrete or asphalt also disallows the development of riparian vegetation. From an ecological perspective they are basically dead zones.

Still, considering several alternatives, I think it's one of the better options. Although it's not cheap to build those, which is a problem in our current capitalitic society