I don't know if anyone else has followed the game, but Black Mesa recently finished adding the rest of the campaign.
It was my first time playing Half-Life, and the biggest compliment I can give is that I wasn't really able to tell the difference between the Valve developed content and the new stuff on Xen. I was sure the long jump was some dumb fan shit (an air dash? In a half life game?) but i was totally wrong.
It's easy to see why the game was a big deal when it came out, and it's a damn fun campaign even today. Well paced, not bogged down with exposition. It's basically just DOOM, and where HL2 picks up is far more interesting, but it's all executed very well.
Pleasantly surprised the retooled Xen chapters didn't add a ton of runtime, as well. It ended just as I was hoping it would, and the final boss was decent enough. Technical problems are still present that mar it slightly. (Got stuck twice and needed to reload save, Xen is as demanding as a modern game somehow when it comes to performance)
As a fan of the HL2 games I could heartily recommend it to anyone interested, especially for the sale prices theyve been offering. Had a great time.