Campaign: too short, "detective part" boring.
A friend of mine and I like to play Halo on cooperative. We've finished both Halo 2 and Halo 3 together, so you can imagine we were pretty excited for a new campaign to play through together. When I purchased Halo 3, he'd come over about once a week and we'd play for a few hours. It took us about a month of this to beat the first one... it was a pretty good and lengthy quest, and there was a lot of parts we wanted to replay.
Halo 3 ODST took us about two nights of this. The campaign is far too short, and ends far too abruptly. There was probably only about two parts throughout the game where we found ourselves truly enjoying the experience. As well, the scenes played as the Rookie were intensely dull. There were no huge firefights in these stages. I understand they wanted to add an element of detective work and searching... but it was nonetheless far too boring.
Another gripe was just how dark the game was. We were constantly using the visor to see better. While it's neat to outline enemies in red, and points of interest in yellow, I find these effects (like detective mode in Arkham Asylum) dulling some already impressive graphics. It just doesn't pack the same oomph. Of course, the lighter stages of the game were bright enough that the visor wasn't needed, which was rather welcome by that point.
All in all, the single player campaign plays a lot like Halo 3, that much is obvious. However, it just seemed to lack some ofthe magic that made it so enjoyable for my buddy and I. It didn't seem necessarily "tacked on", but more of a collection of segments that were on the cutting room floor during the making of Halo 3. They almost should have stayed there, or the ODST campaign should have remained a downloadable expansion. I know, I know, everyone says that.
If you're not a big Halo multiplayer, and instead just want to check out the campaign, I can't recommend this to be worth the price of purchase. Rent it, burn through it in a day or two, and be done with it.