The game released in late '04. You've likely already played games that have been influenced or pushed by HL2's innovations. (facial animations/lip-syncing, physics quality, gravity manipulation, etc) 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.

But then there's the chance that you're one of the minority who simply isn't into the series. That's ok, too. Not everyone was into HL or HL2 when they came out, but we can't deny that their quality has earned them their esteemed status. Both HL games were state-of-the-FPS-art and pushing boundaries at their release.