I would consider myself a fairly decent player. Considering I play it 3-4 hours a day it would be pathetic if I wasnt fairly good at it.
Currently my clan is 2-1 in CAL-O and 2-0 in matches I have played in.
But yeah CS:S owns all other games. I must say that it is fairly important to know a few tweaks since Valve doesnt have the best default settings, especially for hitboxes. Another thing that also is equally important is to have a good server that has a ticrate of 66-100 otherwise you wont get shots to register.