I've played one version or another since the early days of counter strike. My childhood was filled with maps like amazon and dust. When I was 13, 14 and 15 I used to hit up the local LAN center fairly frequently and counter strike was the game of choice.
In Five+ years, not much has changed. As soon as I got Adsl2+, one of the first things I did was try and download counter strike - unforunately it's no longer free if you want to play it over the net. If you do you'll need to buy the game from $19.95 from steam (here's the link).
The reason for this is counter strike source now uses the steam engine to run some of it's more finer points. Like when you shoot random objects - they now use physics and what not to plot the 'real' course of the object.
Using the steam engine adds little to the game play - unless you're a physics major/like shooting random stuff on the ground.
Regardless of the same cost - this is a must buy for everybodies gaming library - but if you don't have a half decent internet connection then you might want to save your pennies instead.