So yeah the first two missions are kinda bad compared to the old SS games, the maps are really closed in and linear and you're rarely confronted with more than 5 or 6 enemies at a time. I did enjoy all the little jokes at other games expense though, having Sam call all the marines Rodriguez and being chastised because they wanted identities of their own was a highlight.
Once those two missions are done with though (took me about 40 minutes on Hard) the game really hits its stride and feels much more like The First/Second Encounter, it starts off by throwing small (20+) groups of the basic soldier zombies at you then ramps up to a group of the big cyclops things that charge at you before sending you into a passage where you get attacked by what was probably more than a hundred kamikaze guys, some skeleton horse things and the first bull alien of the game. You then face one of the rocket launching walkers as a boss fight and move on to mission 4 where, I assume, shit gets real.
From what I've seen of it so far this game is basically what Duke Nukem Forever should have been, it's loads of fun, true to the originals (aside from the very start) and doesn't take itself seriously at all. I can only imagine the co-op is even better and the competitive multiplayer should be decent though I haven't tried it yet.