The GoldSrc engine was the best at the time Half-Life was released. HL was the game that pioneered the free-aiming style of shooters that followed it, and it delivered superb action and a great story to boot, not to mention some great puzzles. Of course, it looks quite crappy now, all because the GoldSrc engine is derived from the ages-old Quake II engine.
That being said, the trailer for the mod promises some awesome action. Can't wait to play it.