Half Life: Opposing Forces is genius, the alien weapons alone are cool enough to play it, and then it has some cool parallels with
Half Life itself. Excellent game/sequel...
Half-Life: Blue Shift? Not so much. It really just seemed like they threw it together to appease the people asking for more, and it's really short. No bueno, unless you're desperate to play more.
Half Life2 is a decent game, but I really thought it had some dull moments, especially the boat trips through the waterways - I was begging for the end of that. And those sequel episodes are fun, but whichever of them has that marathon sequence where you have to destroy all those giant walkers - I don't care if I ever see how that game ends, honestly...