Finished the main story of Portal this weekend and loved it. Everyone complains about how short it is, but it does have various high-score and advanced modes that unlock afterwards. It’s best that it doesn’t overstay its welcome like most games this generation.
That said, here’s some interesting metacritic shenanigans: Orange Box averages a 96 on metacritic (PC). However, separately, Episode 2 averages 91, Team Fortress 2 averages 93 and Portal averages 88. I have no idea where the extra five percentage points come from, other than the Valve love-fest. (Half-Life 2 averaged 96 and Episode 1 averaged 87).
Of course, I am assuming that quality is independent of price, which may be a false assumption.