If you get to write all your business logic in F# and there are no needless sources of headaches, I guess that could be pretty nice if you already use ASP.NET.
The main headache-inducer I've encountered is that to really get the benefits of F# I'd like my model objects to be immutable records, but the ASP.NET MVC model binding infrastructure (along with the binding/serialization/etc. parts of a lot of other .NET frameworks) assumes mutable model properties (and default constructors). You can create a custom model binder, which works but isn't 100% satisfactory for other reasons.
The example gets around this by just using LINQ to SQL generated C# classes as the model, but that's a big compromise.
There was a neat article a while back on building a Starcraft bot using F# for the AI and a C++/C# stack for interfacing with the game at a lower level.
Boo + ASP.NET is also really quite nice. Python syntax with static (inferred) typing and syntactical macros, with a nice base framework. While it hasn't replaced Python+Pylons for me when it comes to rapid development, it's really quite enjoyable for high performance stuff.
The example gets around this by just using LINQ to SQL generated C# classes as the model, but that's a big compromise.