While I think some of its sequel content rides the wave of their brand to be seen better than it actually is, the original Half-Life is a classic for a reason.
While a tad lengthy and occasionally crippled by strained resources pitted against enemies that absolutely won't pull punches, what's undeniable is that Half-Life is NEVER boring. It's excellently paced and extremely varied, full to the brim of memorable and only occasionally frustrating encounters (looking at you, machine gun bunkers). While some of the weapons get a bit gimmicky, especially towards the end, unlike Opposing Force or HL2 they're never shoved in your face as the hot new thing, just dropped in your path to play with if you like.
The final trip to Xen at the end of the game apparently has a reputation of being disappointing, but personally I didn't mind it at all, and the healing chambers were a welcome relief from limited resources pitted against two different armies. My main issue with Xen is at that point the game had been going on for about 12 hours and I just wanted it to end already.
Other than length, the biggest weakness of Half-Life is probably its plot, in that for most of the middle section there isn't one. The game starts off with a nice slow boil, giving you a brief tour of Black Mesa before everything promptly goes to hell, building up from a survival horror feel against incoming aliens before gradually ramping up into an action game as the HECU marines go from merely killing everything and everyone in sight to targetting you specifically due to your stubborn refusal to die. By that point what little the plot has to say patters out until the third act, and all you're left with is an endless series of action and platforming sequences with no context besides "this is the way forward." They're not BORING, I must emphasize. It's just that they keep coming as Gordon Freeman consistantly fails to find the exit.
If this seems like a mostly negative review, I'm rather critical by nature and it mostly stems from my feelings after finishing the game. As I said, its two chief issues are exhausting length and lacking motivation besides "move forward and don't die." From a purely gameplay standpoint the game is brilliant, full of sprawling setpieces which are linear enough to avoid getting lost but nonlinear enough to give you plenty of corners to explore for loot, and for as long as it is, it's a miracle it never felt repetitive.
While I think Blue Shift is an all-around tighter package, Half-Life has absolutely earned its place as a cornerstone of gaming history.