It is free to play and a lot of fun, it runs flawlessly in Linux.
Installing Steam on Linux. There is the easy way and then the way I prefer.
The easy way is to install PlayOnLinux and just follow the prompts once you start the application. This should install all the needed WINE applications and dependent packages.
Once you start PlayOnLinux you can now just choose for it to install Steam and everything should go without a hitch.
If you however find something missing or that WINE complains that something additional is needed, use winetricks to install what is missing. The biggest problem is usually the Tahoma font that is missing or not installed propely, so just install it with winetricks:
Start up Steam and get your game on!
If you like the old school way to install Steam, then this is the way to go about it.
First install WINE, now this can be done by searching for it in your distro package manager, so I'm not going to cover this since it should be straight forward.
Just as a precaution start up winecfg once to create the registry and directories to get everything going.
Additional packages that you may need and find useful for other applications as well is cabextract and wine-Gecko. Search for them in your distro package manager and install it as well.
Next, download the Steam installer and issue the following command in the download directory and follow the instructions that follow:
wine start SteamInstall.msi
Again, once this is done install the Tahoma font with winetricks.
That should do it! Happy gaming!