Aww man, memories.!
This was my favorite gaming series when i was younger, bought Half-Life (Game of the Year Edition) in 1999.. and was blown away by the depth of story and atmosphere; i was following the production of Half-Life 2 very closely including the infamous leak.
Pre-ordered the HL2 Collectors, the source engine was something to marvel at in absolute awe! An amazing physics engine in combination with intricate puzzle systems throughout the game; i miss this in first person shooters.
Unfortunately i never got into the episodes, i think if they had released the episodes as a single release under HL3 then i may of. I did get sucked into counter-strike in my earlier days, along with TF and TF2, opposing force (fantastic game) and Blue Shift.
Portal was a nice surprise, and i already have Portal 2 on pre-order. Valve for me is one of those companies that i can be assured will come out with quality games, that are on the leading edge of graphics whilst still keeping in theme with the classical FPS action games that required the player to 'think' and 'solve' their way through the game while blowing shit up! :D