I've only finished Act 1 in campaign, but I've got about 15-20 hours in multiplayer, and it's probably the most frustrating of the trilogy.
The Lancer is insanely overpowered, it slows you down, and hits like a truck. The Retro Lancer is difficult to use, but hit's harder than the Lancer. Hammerburst, as always, is nearly useless. Gnasher shotgun is good, but if you're against someone with a sawed off, your up a creek without a paddle, unless they waste their shot.
As for map based weapons, the one shot is hella-fun. Sniper rifle can be awkward to use if your used to not using the scope from GoW1.
Overall, much better than GoW2, but it doesn't have anything on GoW1.