Well I have just finished Half Life 1(I am literally about to start playing 2 this is a coincidence) and its 2 expansions for the first time and I will eventually download the community port of Decay as I have enjoyed them greatly. While you will not have the wow factor that the original people to play it had as a lot of the stuff has been done again even in Half life 1.
The games I have found have been a very solid enjoyable experience thus far but not the Jesus they are hyped up to be but you know I think everyone at this point knows not to believe the hype about anything. No matter the movie, game, book, tv show or whatever if you go in believing the hype you will not enjoy it.
So I say if you so choose to play it see what the game is like for yourself and ignore people saying it is shit, mediocre or good as this will pre colour your opinion of the game or expectations. Regardless, you can still see why the games can be considered timeless and are a quite different experience to today's FPS the first and its ilk in particular are more like the corridor shooters of yore like Quake. Well that's my opinion anyway.