Blackmesa Fallback

This is a pretty neat mod.  It takes place in Black Mesa and puts you in the role of the soldiers sent to kill some scientists.  There's a mixture of new content along with textures from Halflife and models from Counterstrike Source.

A friendly chopper attacking hostiles.

The first third of the game has you entering various areas with your fellow squadmates.  There are no puzzles in this section of the game.  It mostly consists of 'missions' that have you opening a door to a building, clearing out an area of enemies, and heading back to your vehicle to proceed to the next area.  There are about five of these missions and I thought they were quite boring, and I was about to quit.  Then the real fun part of the game begins.  It plays quite a bit like Opposing Force, with exploration, puzzles, and fighting new enemies.

The textures are from Halflife and are really low resolution.  This is really distracting, but it doesn't detract from the gameplay.  Most of these textures are on outdoor buildings/walls, so you won't see them most of the time when you're inside.  There are some new enemies, which are pretty cleverly implemented.  There are some pretty intense firefights throughout the game, with two or three really large-scale battles involving tanks, alien flyers, jets and massive explosions.  It's really one of the more memorable moments in the game.

Look how low-res those tank textures are!

The level design is pretty good.  Some area are blockier than others, some have more unnecessary curves.  You will visit some familiar locations from HL1, but they've been changed slightly to keep things fresh.  There are some destructible buildings, but they were really wonky, at best.  If you shot out three walls from a building, the top floor would flop around instead of drop realistically.  Some of the outdoor desert landscapes were impressive.  Some of the tanks, however, are really ugly.  They are made from BSP and use really low-res textures.

Worthless squadmates.

There are a few bugs in the game, such as closed captions not matching the voice-overs, level names not up-to-date, and weapon icons not matching the weapons - the authors didn't bother to edit any text files.  Some funny buggy moments in the game were when I accidently fired on my squadmates, and they all turned on me. I ended up having to lay them busta's down.  It was in a pretty open area and there wasn't much cover, but they weren't much of a challenge and I was playing on normal.

TL;DR  Fun, unpolished with minor bugs.  Old-school mod with massive ground battles.  Not a classic in my book, but still a great mod. 

Scene from ending sequence.  There's no real plot in this game, so  it can't be a spoiler!