testing nixOS for a couple days now, have been amazed by it!
any one out there, who has tried it?

Looked at it many times, never tried.
While it's cool, I don't see any particular reason or roles it would fit better than what I have. (freebsd & Gentoo for the most part)

@lotd the biggest benefits are for me the functional approach to system management, rollbacks have never been so easy, and you will never have any version conflicts of parallel installed versions of the same program.

Yes.
I have noticed that, but as I mentioned, haven't got anything I would see it fit any better role..

I haven't had any real use for rollbacks so far albeit btrfs does provide the functionality and multiple versions isn't much trouble with eselect & portage on Gentoo.. :)

For the functional configs part, I honestly haven't delved much into that :)