Having used F# in a professional context (ok - I wasn't supposed to, but technically I got paid to do it), my general impression is that it's slightly better than C#, but not so much that it's really worth the effort to move towards it. Ask me in 2005 and my response would be totally different, but basically C# already co-opted most of the nice features F# brings to the table, so the only real downside of C# is that it's slightly more verbose and the type system isn't quite as good. In a vacuum I'd rather use F#, but all things considered C# is (almost) as good and using it brings a lot less friction.

Not to mention that it sort of brings a bit of weird legacy cruft from OCaml. Granted C# also drags along some annoying legacy cruft from Java, but using F# on the .NET stack always felt a little bit alien. It was nice, but it definitely didn't quite "fit". How to write "standard" C# always felt pretty simple, but I could never quite figure out what "standard" F# should look like, and it generally clashed a bit with code from other languages.