Episode 2 is Genius
EP2 has one of the best physics engine I have seen in a game and graphics engine has improved a lot but still playable on older machines. New characters emerge and everything seems to work in tandem. Its like planets revolving round the sun. SERIOUSLY. This game could be the only game that uses each core in my quadcore processor because I did not see any glitch from the physics effect which was flawles and it RUNS WELL with over 100fps.
With my system the game was crashed 3 times but that did not keep me from enjoying the game. Playing this game in 5.1 surround is a rewarding experience. I believe that these guys at valve are one of the best in the business. I hope they still support PCs with all other people jumping ship. Worth every Penny.