No they're crap. No one plays them.
I can't believe you even have to ask

I know "there's no such thing as a stupid question" and all, but I just can't believe it!
CS 1.6 is my favourite, it feels more "pure" for me as it's the first one I played so is the game I learned to love. A lot of people don't like the fact that it looks really very dated now, especially people who played CS Source first. To me, 1.6 feels smooth and streamlined, without anything getting in the way of the core gameplay. Playing CS Source, in comparison, feels like I'm playing through a "window" so to speak. It's hard to explain.
CS:S (Source) is the real sequel to CS 1.6, any others you see on steam are just a bunch of crap

It uses the source engine from Half Life 2, which brings much better graphics, sound and physics. There is widespread debate about which has superiour gameplay, I've noticed that people tend to like the one they got in to first, as then the other seems to just "feel wrong" to them.
It's one of those games that you can really hook your reactions/muscle memory into. It can also get extremely addictive if you play with a regular crew and use VOIP.