I have 46 hours recorded in single player mode on Steam and the campaign only takes about 11 hours to beat. This game seamlessly combines old school running and shooting with new school collecting and achievements and it is a tonne of fun.
Favourite weapon has to be the Gauss cannon. It fires a high damage accurate single shot which works great for running and gunning. You can't afford to stand around waiting for the automatic weapons to whittle down opponents. The Gauss cannon takes low level enemies out in a single shot even the really annoying shielded guys. The siege mode upgrade path allows it to charge up a massive area of effect shot. In my opinion the most powerful weapon in the game apart from the BFG. The only disadvantage of the Gauss cannon is that it does some much damage that it kills enemies before you can pull off glory kills.