In general? It is a strongly typed functional language with excellent type inference that isn't lazy and support imperative programming. I think (and this is opinion) that it much easier to reason about an program written idiomatically in a dialect of ML (such as F#) than one written in idiomatic Haskell, basically just due to the lack of laziness. I also think I prefer Haskell to the ML variants, but I also found the hump which one has to get over in order to be productive significantly lower.
.NET is a good technology if you don't mind being somewhat tied to the Mircrosoft stack: it's well designed, reasonably performant, strongly supported and relatively stable. It has good integration with other Microsoft products, good tooling with Visual Studio and an extensive set of libraries. (The libraries are what would probably tie you the most strongly to Microsoft.)
Functional programming is a much deeper topic, so I'll give my condensed and unabashedly biased summary: functional languages tend to be more expressive (and, transitively, more productive) while producing code that's easier to support--F# has a good type system, and functional programming leads to much lower coupling than OO much less procedural programming. This happens without sacrificing performance too much. Sure, you won't get C speed (or even C# speed), but it's more than good enough for most tasks. I'm pretty sure F# is at most a couple of times slower than C#--much faster than the popular dynamically typed languages! I'm really only familiar with OCaml and Haskell, both similar languages, and they can certainly produce fast code.
So if you're convinced by functional programming and somehow tied to .NET, F# is the perfect choice. If you're not tied to .NET, I would recommend Haskell or maybe OCaml instead.
It's not just tied to MS, it runs on every major platform thanks to Mono see the left bar on this site http://fsharp.org/
If there is an equivalent in any other language than F#, do tell.
Gosu was actually the first language I ever used at a real job (well, an internship), which is not a common distinction :P. That also means that my current OCaml internship actually uses a more common language! I bet none of the other interns here can claim having professionally used a language markedly less popular than OCaml.
A couple of years ago whatever the successor to LINQ to SQL was at the time, was not available/not implemented (maybe Entity Framework?)
Anyway, curious about doing equivalent of Scala + Play + Slick on Mono, which I assume is F# + .NET + Type Providers/LINQ to SQL.
I'm skeptical that Gosu's equivalent comes anywhere close to: http://vimeo.com/user18356272/review/66548723/a0d91288e1
It's fucking ridiculous, look at what is happening there and ask yourself, does a similar feature exist in my primary development language?
As a Scala user I cannot answer yes.
The API for actually using the open type system was quite awkward and undocumented back when I used it, but it's probably improved by now.
My main language is probably Haskell or maybe OCaml. We don't have anything quite like that, but we can get quite far just with Template Haskell or Camlp4. E.g. I'm sure I could implement the regular expression example with a Template Haskell quasiquoter. Maybe not quite as slick, but certainly serviceable. I also don't know about tooling support because I'm still using Emacs for everything :P.