I tiptoed around this a bit, but I think F# is great. I have little experience with it, but I work with someone who has been using it pretty extensively for a bit and we've talked about it a number of times.
This, however, is where the environment/culture issue (that I also danced around) comes into play. In most .NET shops it would be extremely hard if not almost impossible to bring in another programming paradigm like functional, let alone mix it. This is perhaps a bit ironic considering most of the same places use javascript, but if they could write C# instead of javascript on the client side, I'm certain most would (in fact when ASP.NET first made this somewhat possible it was the norm until people got bitten by all the pitfalls).
From what I can tell the biggest issue you (and many others have) is the environment/culture issue. The stuff you see or some of the ways that you are expected to code with C# in many .NET shops will make you want to claw your eyes out. (And though I don't have any first hand experience, I reckon it's the same way in Javaland.)
F# is a mighty fine language but introducing that into an established shop just isn't going to happen. It's personally my favorite language but there is a very steep learning curve coming from C#. For my projects I use Windows & Linux, C#, Python, JS (client), and F# all over the place. I keep the programs small with a service orientation so it's easy to switch languages.
Anyway, all frameworks have their problems. It's hell to make a program of any appreciable size in Python. Javascript/Node turns into callback hall of mirrors. Avoid creating a mammoth framework with these and it all works out great. And while it doesn't have everything in the world like the Python ecosystem, .NET OSS ecosystem, which is really coming around. And Microsoft itself is encouraging exploration of other frameworks, e.g. NodeJs and WinRT's first class Javascript support.
My take is the problems usually aren't framework/tech related. It's about people and process. Diving into a new framework really isn't about the tech, it's about learning the tech, and thereby giving your brain some much needed stimulation.
