I feel Half-Life could have had a lot of potential, but pissed it all away by making things too linear and scripted. The maps are boring. Nothing feels spontaneous, or unexpected. Even the fights feel over scripted. End result pretty much kills replay value.
Xen wasn't all bad, the platforming stuff is what ruins it.
Opforce and Blue Shift were more entertaining, but also suffered greatly from the original game's primary issue.
I refuse to try Half-Life 2 because of all I've heard about the "loiter and we will kill you with grenades from nowhere", and the same issues that kill the first game.
Seriously, if they were just gonna make me play SmashTV in fps form, they should at least be upfront about it. (come to think of it, that was Serious Sam. Which was crazy. In a GOOD way.

)
Unreal differs from Half-Life in this regard because Unreal feels and behaves more like Quake. The level design is also much better for replay value.
So yeah. HL is the best game evar.

Bitter frown here? Very much so.