Something like 7 out of 10 of the top most popular games can be made to run on linux. Like ut2003/ut2004 run natively in linux, stuff like BF1942 can be made to run thru WineX.
WineX will knock off some performance though because of the translations it has to make to have windows version executables and directx stuff translated to Linux executable formats. Some games can be run at near windows performance levels, most games you going to see some performance hit. Varies from title to title. Some games are just unbearable.
I've run a couple games thru WineX, but mostly I try to stick to linux games that have been ported. Mostly I play quake3 mods, because I like the competetion of online stuff.
Otherwise I just have a Nerdtendo (as my roomate likes to call it.) running thru a Tv capture card(my TV is sitting disused in a corner of the room right now). Games are cheaper and are easy to find. (I go down to the used game shop and pick up a couple old titles time to time, or hit up Ebay)
Reasons for using Linux?
It's cheap. It's secure. It's stable.
You can get tech support for it, just like you can thru windows. But it costs then, just like windows.
No unneccessary pop-ups, pop-unders, viruses, worms, spyware, adware, and all sorts of crap that you pick up and have to regularly clean from windows stuff.
It's easier to run internet services from home. Like a personal website, for instance. I like to use ssh to connect and transfer files to and through the various computers that end up using over a week or so. It's nice to play around with different things like that (but you do have to be carefull, a badly configured Linux box is as easy/easier to hack as a any Windows box.)
It's nice to have if your a computer enthusiest, even if you don't use it as a desktop. It's a very powerfull tool to have at your disposol. Also if your into developement and like to hack away at programming time to tme then Linux is a very friendly enviroment.
Customization is also nice. You have a much wider veriaty of choices to what programs you want to use or need to use. Most of them are free and open.
It's definately daunting to use if your use to being a fairly knowledgable Windows guy.
On badly supported(hardware makers supporting linux is the real problem) hardware, linux can be a nightmare to try to install, and it will never quite work correctly. But that's becomming less and less as time goes on. On hardware that is supported well, a install is actually somewhat easier then a windows install.
It's considured more difficult to use by most people.
But it's more difficult in the same way that a big tank-like catapiller tractor is more difficult to operate then a lumbering SUV with automatic cruise control. If getting from point a to point b is most important then the SUV (windows) is going to be much better/easier (except for the occasional faulty tire-induced rollover) for you, but then again have you ever tried to dig a trench or tried to knock down a building with a SUV?