The single best remastered game I've ever played. With the release of Xen, this is effectively two games:
Part 1 is an exceptionally well polished recreation and enhancement of the original Half-Life Black Mesa incident.
Part 2 is a completely redesigned and beautiful Xen world.
The first part is faithful to the original game in plot, approach, and tone. We all have friends who never played HL1 before. Have them play this. It's essentially the same game. The developers did a wonderful job weaving modern game mechanics, a new cast, and HL2 plot elements into the original masterpiece.*

Where Black Mesa really shines though is Xen. Yes, that Xen, the original jump puzzle hell is gone. The new Xen is where the developers really got to stretch their creativity and programming prowess. This is where the influence of modern video game design really shines. New Xen is absolutely beautiful, uniquely it's own environment yet calling to mind the alien artistic designs of Subnautica and occasionally DOOM:Eternal. Xen, like all of Half-Life, is still essentially an action FPS with puzzle elements. The puzzle design is well paced, and feels as natural as anything would on an alien planet. Compared to the original Xen, Black Mesa developers really benefit from modern puzzle platforming design, and none of the puzzle elements would be out of place in a Portal or DOOM game. The new Xen is much larger, nearly a third of the entire game now. Half-Life is one of my favorite games of all time... and this new Xen might just be my new favorite part of it.

How many games can you get these days that perfectly recreate the emotional, rose-tinted, and memory faded experience you had with the first game two decades ago, while also giving you a whole new chapter that is twice as long as most CoD titles? The game is worth buying and is really THE way to now introduce new players to the Half-Life franchise. Give it a go, and be prepared for unforeseen consequences.

*You can almost feel the developers pain when they perfectly recreated the weapons in-game to look exactly like their real world counterparts, but then decided to keep HL1's occasionally strange shooting design decisions. The pump shotgun still fires two rounds at once for... reasons.