I follows the yesod web framework for some times now. And I believe it reached the point where you should really consider to use it. But instead of telling you why you should learn Haskell and use yesod, I prefer to talk about new ideas used by yesod I didn't saw in other frameworks.
Furthermore, if you use say 10 widgets each with a bit of CSS, yesod will create a unique and compressed CSS file. Except if you expressed a need to change the header by using different CSS.
In standard routing system you have for each entry a couple: regexp -> handler
The only way to discover the right rules is to match each regexp to the current URL. Then you can see behaviour such as, if you change the order of the rules you can lose or win time.
On the other hand yesod compile the routes. Therefore it can optimize it.
Why you should use yesod. Just saying from a very subjective point of vue, from what I heard, Haskell is a node.js done as it should be.
1. Speed. This is just astounding.
2. Haskell. This is certainly hard to learn but it is just incredibly awesome. If you want to make you a favor. Just learn Haskell. It will be difficult, far more than you can imagine. It is very different from all other languages I used.
3. Good ideas, excellent community. I follow yesod from some month now and the speed at which the project progress is incredible.
If you are a haskeller, I believe you shouldn't fear the special syntax imposed by the standard yesod way of doing things.
Until here I believe it goes in the right direction. Even if I believe the real future is by generating HTML pages from the client (using javascript) and server limited to serve JSON (or XML, or any object representation system).
I cannot stress too much about how I believe Yesod is good.
One of the most important point is the responsiveness of the community. It is very active. It is Greg which the first had given a way to [deploy Haskell code to Heroku](http://www.yesodweb.com/blog/2011/07/haskell-on-heroku).