Awesome flight 343i! My experience has been really smooth in general, maybe just a few things popped up:
-Campaign has a few visual bugs in anniversary graphics mode, which I submitted a ticket for (most/all of these appeared on The Oracle in the early sections). I noticed the energy sword model vanished looking at the flood container at the start of the level, a door leading out of the first area with the heretic hologram didn't render at all and I could see through it, and the sentinel beam's beam appears to be off-centre. There was also some extreme transparency in the first corridor where you hear the flood beneath you but I'm unsure if that's my hardware or the anniversary style? They remain obscured in shadow and smoke in classic graphics.
-Multiplayer, I had the occasional issue finding a match (slayer modes, non traditional, swat/snipers, and flag/bomb modes) but this could just be general matchmaking (it works 99% of the time but rarely I find 10+ minutes of waiting with no connection and then, once I leave matchmaking and return, it just finds a game soon after). I was dropped from a couple of games but nothing major, and in one match on Midship, someone killed me with a sword, causing my body to drop through the floor and into the space skybox with extreme speed, only to rebound back to the map and have stretched ragdoll limbs bouncing around, all during the death animation. No footage or screenshots unfortunately.
Otherwise, fantastic to see Halo 2 run so well on PC!