Highly recommend this one
It consists of 12 Chapters, but overall, is just as short as Blue Shift
What i liked about it, was the Boot Camp. basically it's a typical traning mission, but well, a nicely done traning mission.Had a lot of Army stereotypes and Full Metal Jacket references
Now, unlike Blue Shift the game has a lot of new things added to it:
You get nightvision instead of the flashlight
sniper instead of the crossbow
pistol with a laser pointer instead of the revolver
wrench and a knife instead of the crowbar
2 new alien weapons, one is like a bazooka, the other has infinite ammo(shoots lightning balls)
Instead of long jumps, your protagonist climbs on ropes
1 new Black Mesa prototype weapon, that teleports you to Xen. The location you get teleported in has a lot of loot.
1 completely new weapon, not just an alternative model - a machine gun (M249 SAW)
There are new alien types to fight
Also, throughout the game, you will encounter your comrades, the most important ones are medic and engineer. Medic can patch you up, while the engineer can drill through doors.
And you would probably want to buy it for the plot
Though just like with Blue Shift, it doesn't really collide with Half Life 1 plot, you see Freeman once and that's it, but unlike Blue Shift, it has some important stuff to bring to the table