First, I find this software absolutely amazing, thanks! I'll definitely go buying it once the problem I mention below is solvable.
The only game I play is the half-life mod "Natural Selection", and being able to play it on my new iMac would allow me get rid off the old PC completely.
Steam and/or HL installs went fine, graphics are OK, single player mode runs fine. However, I got the following problem when playing online via Steam: the moment I try to connect to a server, I get a refusal because "Your version does not match the servers".
This applies to both the Natural Selection mod as well to Half-Life itself. In fact I checked that the Natural Selection version indeed is the latest (3.1). and also I tried copying the whole HL subdir tree from my working windows installation into the new location, with the same error. I also made a fresh install by not installing HL directly, but just installing Steam first and then installing via Steam the game HL (and later Natural Selection by hand).
I it may well be that this is more a problem of the Steam/HL installation and thus may also affect the Counterstrike mod. Did anybody else encounter such a problem?