The only 'case' against it (if there is such a thing) is that most modern, professional developers use different tools (as well as different tool types). F# brings some unique tools to the game, and those of us who embrace them will find our respective, specialized talents useful for other programming tasks -- especially those tasks involving analysis and manipulation of large data collections.
What I've seen of F# truly amazes me. Every demo leaves me grinning because F# strikes me as an advanced edition of what I remember from 'the good old days' when functional programming was much more common (probably more 'old' than 'good' to be sure, but such is nostalgia).
