WORTH THE WAIT!

Seriously though this game takes the mod - which was ALREADY brilliant - and makes things even more lovely. Better visual effects. Cleaner textures. Way better UI. Remarkably better AI and QOL. Better sound quality, FANTASTIC soundtrack, and charming vocal talents fleshing out the world. Just... augh, I LOVE THIS GAME. And if you're a fan of the Half-Life series, you will too.

Now, let's pretend you've never heard of this mod or half-life. Is it still worth your time?
Absolutely, yes. This is not only a competent shooter, but one of the best in it's field. Though not the most modern, it is extremely well put together, it handles magnificently, it has wonderful ideas (both from Valve's original and from the dev team), it's very well-characterized and it's one of the keystones of gaming upon which an era was founded. You might not know it, but you probably need this game in your life. The science lab levels are a solid jaunt, but quite frankly, the Xen levels are some of the best experiences I've ever had with a first-person shooter.

"Is it worth shelling out $20 when the mod is free?"
In every respect, absolutely. The mod has a fair number of bases covered, no question, but this uses a much newer engine, has a lot of tweaks and upgrades to the mod, and is improved in virtually every aspect in ways both little and huge. Not to mention, my god, these guys have put more heart, soul and brilliance into this game than most overhyped AAA devs ever bother to.