There is a reason the majority of the big tournaments still are held in CS1.6 and not Source, according to many people. I believe 1.6 is much more a game of aiming, timing and skill than Source is - simply because the game is better engineered for the internet. This better engineering gives a lot more precise shots - if you shoot someone in the head you actually make a headshot. This can be seen compared to games like COD4+6 and BF2 where it's really all about luck whether you make that kill or not.
In BF2's case that doesn't really matter since the game is mostly about vehicles anyway - but for COD4 and COD6 the horrible network optimization is extremely obvious unless you play on LAN, trust me

.
I find, at least, that in CS1.6 I land much more shots than I do in Source, especially those aimed to the head or upper torso. Oh and let's not even talk about the grenades in Source... Come on! It's made to be realistic, but I would like to see the guy who throws a grenade that far in real life

! And also, you never know where the grenade hits, you can't make those really important flashes, smokes and grenades through tight openings or doors like you can in CS1.6.
What I really like with Source, though, is the realistic rendering of the maps. That is way beyond what 1.6 can achieve, and way beyond a lot of today's games. That's why I, at times, love to map for Source - it's simply great!
These are just my two cents, most people probably don't agree with me. But yeah, I'm up for a 1.6 FightNight!