When Half-Life was first released audiences were stunned and amazed at how a First Person Shooter can be more than just clear level 1-10 and win the game but can be more of a narrative tool. I too was one of those people and absolutely loved it. Even for such an old game, the enemy ai is excellent and the gameplay is a great mix of ambience, action, and story following the incident at the Black Mesa Research Facility through the eyes of the inconspicuous Gordon Freeman. One several parts that amazed me were the ai of the enemy soldiers as they used grenades to flush you out and sometimes left grenades at their feet when retreating. At the time, the scripting blew me away since stories in first person shooters were never portrayed but it might not be anything special since all first person shooters these days use it.
In this day and age, deathmatch multiplayer just doesn't cut it. I'm not a big fan of deathmatch, especially when it isn't made properly, and half-life's standard multiplayer only includes deathmatch. But there are many more multiplayer options you can take with its massive modding community. Such great mods that I would recommend would be the total conversion mod of Aliens vs Space Marines in Natural Selection as well at the multiplayer co-op mod Sven Coop. These are both great fun and will give you hours upon hours of gameplay.
Another feature thats pretty great is that you won't need to lug the cd around with you anymore. I bought the game quite a while ago when CD's were the popular game format. Now with the digital distributor Steam, you won't have to find your cd and cd key everytime you want to install it. Currently, I've probably reinstalled half-life around 4 times now.
Half Life is an excellent buy and for only 20 dollars you really shouldn't miss out on what started the phenomenal Half Life story.