There are a number of Haskell libraries for developing websites (Happstack, Kibro, Hack, Salvia, Turbinado); the problem is that it's hard to know which ones are good. Many of these libraries are quite young. I expect that the situation will become better in the next couple of years as these libraries mature, and become clearer as these libraries become better documented.
In the meantime, I would say that Haskell is a fine language for writing web applications, provided that you are willing to spend some time getting to know the libraries and perhaps improving them when they fall short of your needs.
