The same great gameplay, from a different perspective.
-same great gameplay
-fun new weapons (e.g. chain gun, portal gun, magnum, grapple hook, etc.)
-cool experiencing the first Half Life through a different perspective
-same mysteriously intriguing story
-some new enemies
-some tweaks that improve minor annoyances over the first game (e.g. health dispenser restores health faster, etc.)
-ability to recruit Marine allies and give them orders
-lengthy campaign for an expansion pack
Bad:
-Marine allies can get in the way sometimes, or you must order them multiple times to follow you
-a couple frustrating segments (e.g. the Voltigore's Lair)
Overall:
If you enjoyed the original Half Life, Opposing Force will satisfy your needs by offering you an extra few hours of the same fun gameplay, as well as a few new surprises.