The game released in late '04. You've likely already played games that have been influenced by HL2's innovations. So, at this point, it's likely that the game will have a lesser total impact than it would've had six and a half years ago.

While it still retains its quality over time in the sense that a classic car does, the way in which games technology progresses means you lose part of a game's punch when played too late. If you only lose hype, then so be it. If you lose the innovation of its time, then you're missing out.