Been using C# for a while now for shrinkwrapped winforms database application and web site. We started intermixing F# in the last year. I've found F# to be better than C# in everything except a few fringe cases where libraries have done things specific to C#.

For example, F# has a better ORM framework than anything in C# with Type Providers.... unless you need to support two databases in the same dll. Then when trying to use nHibernate, we found that Nhibernate mappings expect virtual properties, which F# didn't easily do. So we have a C# project with a few C# classes used by the ORM, then a conversion layer into F# domain objects, which we would've done anyway, because ORM classes are terrible at expressing the domain compared to F# records and unions.

Other than a few weirdnesses like that, we've found it to be great and otherwise a drop in replacement for C#.