If you haven't tried it yet, here are the main features:
- Syntax highlighting.
- Indentation. The mode knows where blocks begin and end, and tries to correctly indent your code.
- F# Interactive support. You can easily send your code to fsi and get the result.
It is important you use a recent version of Emacs, as it fixes a few problems. I recommend you to look at the README file for a complete description of key shortcuts. For instance, you'll learn that Alt + Enter automatically adds the ";;" before sending the expression, in the interactive buffer. Now, get fsharp-mode 0.3!