Re: Bug free?
The comment is probably around that functional languages favor functional purity and referential transparency, which generally make for bug free code in addition F# provides a number of tools to ensure issues around null are highlighted as well as adding to a discriminated union creates warnings where cases are not covered.
Sure none of this means you will be bug free and anyone can screw up the simplest of things but it does make falling in to the pit of success easier than the jaws of failure.
I agree that F# is still not mainstream yet however its on the rise going from 69th to 12th on Tiobe
http://www.infoworld.com/article/2610183/microsoft-net/article.html
Congratulations Don