Even Gabe Newell thinks it is shit and looks anguished whenever he talks about it.
Gabe Newell's understanding about shooters nowadays is probably lower than the stock poster in this thread. It's the same man who was jubilant about moss growing in real time, if you don't remember. He's simply repeating memery.
Regarding how "bad" Xen is, well, the only bad things in it are a couple of jumping puzzles, for the obvious reason that first person jumping puzzles are shit. And it doesn't help that Boss fights in Half Life have always been mediocre (the Tentacles barely count because they're more of a enviromental hazard standard level), but both Gonarch and the Nihilant are .... well, "good enough" at best. Valve never got good boss fights in, remember that HL2's final fight is an elevator and two gunships that you immediately blow up.
By itself, the Xen levels are at worst
rushed. There's still signs of the typical Valve care in level design (you can take alternate routes avoiding sentries in the Vortigaunt village, the Gargantua can be stealthed, some creative secrets, Vortigaunts not being hostile in Interloper) but the thing is that the union of
poor visual design and those few shitty puzzles draw all the attention. Most of Xen is fine, the problem is that it looks drab and you have
those shitty jumps, souring all the experience for the less patient player.
Half-Life was great for its time, but it hasn't aged well.
Then show how Half Life hasn't "aged well". Even amongst his contemporaries, Half Life keeps and edge, consider that probably its most direct opponent was
Sin, a fun but deeply flawed game (I like
Sin and its original approach, but it's undeniable that something went wrong at some point in its development, too many ideas and not enough refinement). Shogo: MAD? Blood 2?
QUAKE II? Unreal's SP section is fun, but it's lacking compared to HL. Of course, you can compare HL to its own follow-ups like the 00ies PC shooters like.... HL2, FEAR, Doom3. I'd argue that FEAR has a better gunplay, but again, very few things can compare to FEAR's gunplay, and HL2's combat is a tad inferior to HL1. We should consider console shooters? They're all mostly trivial in design due to engine limitations (I'd freely admit that some levels in Combat Evolved are fun, tho). CoD-likes? Theme park rides, not shooters. Even the "retro" shooters craze of today fails to produce better examples.