megabump.
I am playing this right now.
The beginning of the game is practically a 1:1 clone of the original HL1, with some quality-of-life improvements (no more dying on every damn ladder), visual upgrades, and a better UI.
The end .. well, the end is nearly as big as the rest of the game. While in HL1 once you go to Xen, you have 1 big map and a stupid fight against a boss that stands still, in BM you have several hours of brand new gameplay, totally new maps, a much more fleshed out view at what the Xen dimension's world is like, additional lore, and, well, i haven't gotten to the end yet.
One a point of view, i love it. Not only is it better in everything compared to HL1, it's amazing that you can get this much game for this little money.
One another, Black Mesa, and Half Life, are not shooters; they are puzzle games, platformers, with a bit of gunplay mixed in - don't expect Doom.