I have a confession to make. I got a very late start in the world of FPS. In fact, Half-Life marks the first FPS PC game I've ever purchased and Wow am I impressed. Now I have some catching up to do.
The storyline is great. Not entirely realistic to expect a nerdy scientist to kill his way through 1000 aliens/soldiers, but it's always fun to pretend.
While the aliens were cool, I much preferred fighting the soldiers, thus the game lost something for me once I was transported to...whoops, I don't want to give away too much. Suffice it to say, I'd likely prefer a military-based over a alien-based FPS for my next experience.