I have actually used it since the beta invite popped into my inbox. For those of you who havn't tried it here is a short summary:
I run Arch Linux, which is not supported. Valve only supports Ubuntu and provides the software as a .deb file which contains the "bootstrapper", basically a "netinstall" version if you were to make a comparision to the average Linux distro. The bootstrapper is easily taken apart via a script in the custom installer program that some of the Arch Linux folks whipped up and ends up installed system-wide by default.
This caused some problems for people like me, who are too paranoid to install untrusted software system-wide or even in my own home directory. I gave it a separate user account and denied the installer root access (which it asked for every time it tried to auto-update). It cried and bugged out, but you could run TF2 from day one. As they continued to improve the software they actually listened to the complaints at github (where they keep their Linux issue tracker) and made the software runnable as a regular user. It now resides completely inside my 'steam' users directory and the bootstrapper is long gone from the system-wide install.
If you are like me, and only run ALSA, hating PulseAudio's tentacle guts, you can actually run Steam anyway. They are using SDL as the backend, so when launching Steam you just export SDL_AUDIODRIVER=alsa before running it, and you'll get sound! Even in-game voice is operational, but you still can't permanently disable it to get rid of all the jackasses screaming into the microphones.
Steam itself still uses the look from it's Windows roots, the ugly custom-skinned UI. And it can't be resized on my machine, which runs PekWM. It is also slow as molasses to start, and so is TF2. That might be in part to me using ONLY a 3G modem for my gaming though. The store also works like a charm.
An interesting feature is that you can actually switch between the OpenGL game window and the rest of your desktops seamlessly, with no apparent bugs or performance loss. Faster and more painless than on Windows. This wasn't always the case though, as early versions would switch to your desktop as soon as you got an archievement and completely screw up your mouse input once you switched back. This has been long since fixed though.
The only recent bug I came across was an apparent lack of support for multi-user environments, where I once started the bootstrapper as my regular user by mistake and let it install, thinking it was an regular update. Once it was up I figured what was wrong, uninstalling it and starting up as the 'steam' user, whereas it sefaulted hard. It took several hours and a lot of support ticket reading to figure out that leftover temporary file descriptors left from the first session screwed up the second one. Kinda stupid bug for a modern software, but that's what beta testing is for I suppose.
For me, Valve has really made my Linux experience a lot better. Hat's off to them. Now I just need to find some TF2 servers with players that are as beligerent and offensive as me!