I remember switching to Linux full time in about 2003 when XP SP2 came out. I had been thinking about it for a while, but the update that switched on the then badly configured firewall was the one that pushed me over the edge.
Since then I have always had a Windows installation on my main home computer purely for gaming. For a very long time Linux gaming was purely some open source games that even though sometimes pretty good, where quite limited in their number. And it really wasn't something any of my friends where into. As I have always played a lot of multiplayer games, this was sort of a big deal. There where some attempts to bring games to Linux, but they where always done by third parties that had Linux gaming as their sole business, and they never lasted long. It was sort of a chicken and the egg problem. No gamers on Linux, because you couldn't game on linux. I was guilty of this myself as well.
Suddenly, things changed. There where pretty much 3 factors that together contributed to it. Unity game engine support, Humble Bundle and Valve.
Firstly Unity game engine has made it possible for indie developers to release their games on pretty much all platforms without too much extra effort. This has made to number of Linux releases skyrocket.
Then Humble Bundle, a really good charity concept decided that they will release their Indy Bundle on Windows, Mac and Linux. They actually managed to get a few developers to release Linux versions of their games, just for the bundle. Unfortunately some of these developers have since neglected to update these games, but it's a good start. Humble bundles pay what you want pricing have also shown that Linux gamers are not only willing to pay for games, but they consistantly pay more than users on any other platform.
The last factor, and the one that might get Linux gaming to the critical mass it needs was Valves announcement of the SteamOS Linux distribution. This was not just empty talk either. Even though the SteamBox has not really taken of yet, that is at least partially because Valve has not yet released their reference box. If they are just working on it until it's ready, or they are waiting until the games are there is anybodys guess.
The reality is that suddely it is quite possible to game on Linux. Were not all there yet, in my collection about 70 of 230 games have linux support, but I would also say that about 50% of the new games I'm intrested in have current or planned Linux releases. Of course the whole concept of Linux releases vary quite a bit between games. Some are truly native experiences that work atleast as well on Linux, some are Windows games wrapped in a wine wrapper, and a lot of them are something in between. But mostly it's heading in the right direction.
The bad part of Linux gaming at the moment is however not the games themselves, but the hardware and driver issues. First there are the GPU:s. If you want to game you pretty much have 2 choices, Nvidia and AMD. Nvidia used to be the favourite among Linux gamers, and for many they probably still are. The problem however is that even though Nvidia releases the fastest drivers for Linux, they are horrible messes that you really don't want to touch with a ten foot pole. Linus Torvalds famously said "F** you Nvidia" at his Aalto talk, because Nvidia are so horrible to work with, and it shows. They have a complete Windows mentality about relesing drivers. They release no specs so the open source versions are horribly slow and buggy, and their proprietary drivers works unless you do something like upgrade your kernel and your card is not too old.
AMD however have really stepped up their game. You don't get nearly as much speed as you do in Windows out of the cards, but you can still use them for most things. AMD releases all specs for their cards, so open source versions usually work before the cards hit the market (although sometimes running quite slowly). The proprietary drivers are a lot better than they used to be, but it's a stretch to call them good. They are however quite a bit faster than the open source drivers, and some games get glitches if you don't use them (then again, some games work better with open source drivers).
Another problem is that most Linux distros lack a standard way to handle things like joysticks. Don't get me wrong, joysticks and wheels and such usually work fine on Linux, but they are difficult to configure.
Now it seems that the ball is in the court of the Linux users. The game makers are delivering, now it's up to us to make Linux a game friendly environment.