Sorry to contradict you, broli, but I really liked playing CS 1.5 and, tho it is true that it was played very much by cheaters and ppl with no cd key, that is not my biggest problem. I do not like CS 1.6 or the version afterwards because it is not the same feeling playing them. Maybe I played too much CS 1.5, then I took a break of almost one year from counter and when I came back ppl were playing CS 1.6 for a long time and I needed time to adjust to it, but I wasn't able to do that.
I have original CS 1.6 with CD KEY but it is somewhere in my CD-case since I am playing CS 1.5 on the few servers left. Yes, the biggest problem is represented by the cheaters, but I like it way more than 1.6. Call me old-fashinoed...