hmmm...I can't speak for co-op, but I just beat the game on Heroic (second highest difficulty) and it took me a little over 11 hours (I confirmed this to the exact time because you can actually see how long your films of each level were, and as a result the total game time (this doesn't count time watching cut scenes)). This was single player mind you. I'm planning on going through the game on Legendary by myself soon too.
I can totally see how the game could be a lot shorter on co-op though. And it's a lot easier to just blow past enemies in Halo 3 (meaning, if you don't want to fight them you don't necessarily have to if you can get by them without dying from getting shot). In addition, I really hope people don't play co-op on Normal difficulty because it'll be way too easy (and a lot shorter). I know people will jump all over comparisons to "Heavenly Sword" and the similarities in length, but I get the feeling people beating Halo 3 in 6 hours are either seriously rushing and/or playing on a level of difficulty that is too easy. I could be wrong, and these people just might be God's beating the game on Legendary in that amount of time (I seriously doubt this).
If Bungie made a mistake it was making Normal way too easy for people that have played FPS (especially Halo) in the past, and Bungie themselves have even said that they feel that if you've played Halo in the past, you should start at least at Heroic if you want any challenge. The game does have great Multiplayer, Forge, and Saved Films that really add a lot to the replay value; and the best part is they can all be enjoyed locally, on system link, or over live with all your friends. Halo 3 may have its faults, it sure is fun as hell (bungie did a great job trying to get back to what made Halo 1 great, and I feel they did a great job of that)...and to me that's what matters most.
And to the OP, that really isn't a fault of the game, but rather the crappy DVD drives. I too hate the loud 360 drives, but I can say that the Elite I bought recently has a very very quiet DVD drive (BenQ) compared my launch Premium (Samsung) that I gave to my brother (and still runs without any problems...knock on wood).
The internet is rampant with Halo 3 hate, or bitterness, lately. I understand to an extent why, but it really is a fun game. And the hype will be dying down soon. Fret not, for in a few weeks we'll be in the full swing of the Christmas season and we'll be hearing plenty about the other big games of the season. Yes Halo 3 has been hyped, but in my opinion it lives up to a lot of the hype. I encourage anyone that hates the game to at least try it. I'm sure not everyone will like it (because lets face it we all have different tastes, and if you don't like FPS or in general HALO games you probably won't like it that much); but you should at least give it a chance. I hope all the game comparison stops soon (i.e. Halo 3 vs [insert other "killer" game here]) too and that we can all just enjoy games...ah, I can only wish...