CPU: i5 750
GPU: Nvidia GTX 285
It all runs surprisingly well for me, past the initial effort to get it installed on Gentoo. TF2 runs at ~65FPS, which is only about 15-20FPS less than I get in Windows (but not really noticable to me), and was very stable while playing it for many hours. Serious Sam 3 is probably the best looking native Linux game I’ve seen and runs very well too, I’ve not had any issues yet besides the odd stutter.
I’ve not tested them specifically, but I get the feeling that the updated Nvidia drivers make a big difference, as I keep seeing people with better specs than me getting worse performance on older drivers.
I’ve had some issues with sound, the Steam overlay, and the client updating at some point, but I think these are mostly down to using an unsupported distro and not having all the dependencies/settings that they expect. The only problem left is that it keeps changing my mouse pointer, which is quite annoying. Multi monitor support also isn’t great, but that’s always been the case with any sort of games on Linux for me.