Gaming is OK in Linux. Definately better then it is in Macs, but less then it is in Windows.
Doom3 is native to Linux. UT series of games have native versions.
Halflife 2, Steam, Counterstrike, etc Need 'Cedega' (formally WineX) to run, which is a subscription based software.. 5 bucks a month.
Same thing with World of Warcraft and many other games.
When running in wine/cedega these games tend to have a performance hit.. The code is not as optimized as DirectX is on Windows.
Also ATI drivers for Linux suck, very slow comared to Windows and a pain to install. Nvidia is much nicer, with performance comparable to Windows, even though they are still propriatory and need to be downloaded and installed seperately. (some distros have optional packages for it.)
Some Linux gaming links:
http://icculus.org/
http://www.linux-gamers.net/
http://www.linuxgames.com/
http://www.happypenguin.org/
http://icculus.org/lgfaq/gamelist.php
Generally you don't want to use Linux for Gaming, unless you use Linux for other reasons and you want to play games without dual booting or paying for Windows (or just putting up with windows).
Personally I feel that Linux operating systems are generally better. It's stable, respects your freedom, fairly secure, and very flexible (especially for technically-oriented users). I like the user enviroment more then Windows.
People sometimes find Linux a difficult adjustment. Stuff that they thought was easy in Windows can turn out to be difficult in Linux, especially for a newcomer. Documentation may be confusing and is sometimes hard to find.
I think that it's worth it personally.
If you want to try Linux out I suggest using a few of the many "Live CDs" that are aviable.
Live cds are OSes on a cdrom that you can use without having to install anything on your computer.
two worth trying out are
ubuntu offers a 'live cd'
http://www.ubuntulinux.org/download/
and Knoppix was one of the first popular live cds.
http://www.knoppix.org/
See also:
http://www.linuxiso.org/ for many more cd install images, and some live cds.
http://distrowatch.com/ for distro comparisions.