People keep on bagging Halo SP, and deervedly so in some respects, but I have to say that they crappy moments (Library) are balanced by some very good ones (Silent Cartographer, the tank missions, the final warthog race/track thing). Where the SP shines is in the excellent AI and the great humour that has been injected into it by the sound designers (all the comments by the grunts and marines). Some excellent writing too, and a strong story - something very rare in a FPS.
I just spent three days playing Halo (plus some other games) at a LAN party this weekend and it turned out to be very popular, even amongst those who thought they were going to hate it. After three days we had still had more game-modes we hadn't tried yet, and most of the ones we had played were good fun.
'Stalker' is a great MP mode much like juggernaut where the target is invisible, so you've got one invisible guy going around assasinating people while everyone else is trying to kill him. The guy who kills him becomes the new stalker. We had a great time with this.
One thing I observed was that it was while playing Halo that we heard more laughter and "woah!"s than any of the other games we played that weekend (Raven Shield, Unreal Tournament, Total Annihilation, Rise of Nations and a few I can't remember). This is a great LAN game to be played amongst friends, and one of the most consistently enjoyable multiplayer experiences I have had the pleasure of playing.