Is it? I thought all Source games share more or less the same level of code quality.
Well, TF2 is an exception. All the patches have bloated the game to no end, some people with high-end computers are forced to run it in DX8 because the game won't work properly, the a VERY LARGE percentage of the people running GT2** series cards can't play the game because it crashes the nVidia driver, among many other issues.
I, myself, have managed to get the game running properly after so much time of optimization, and trying experimental configs. But I ran into many issues in the past. Also, since the pyromania update, the game's perfomance has gone down a lot. I used to play the game at 180/200 fps and 50-60 fps on firefights, now it usually tops at 120fps and 30-40 fps on firefights. Certainly not unplayable, I'm quite a decent player if I can say so myself so it's usually not an issue, but high fps helps a fair amount when playing scout or sniper (or against them).