I had a chance to get some hands-on time at EA Redwood Shores and Onslaught is definitely a lot of fun. The mode plays out like a game of Rush, only your squad of four is up against enemy AI instead of other players. You can respawn as long as one team member is left standing and you'll want a good mix of classes in your squad as ammo will run low, vehicles will menace you, and allies will need healing.
This is, of course, the first cooperative mode to be introduced in a Battlefield title. It works well. Extremely well. The game engine, combat, and objectives all work well with other human players. The difficulty levels offer nice challenges for players of all skill levels.
The AI is good and will fire rockets at your vehicles, snipe you, and toss grenades. Enemies will drop kits just like in multiplayer allowing you to change your gear on the fly.
While the maps aren't new, the day-time/night-time tweaks for Onslaught give them a different feel. New achievements and trophies will be available and there will be leaderboards to rank people by completion time.
The only downsides I saw during my limited session is that the waves of enemies seemed identical between playthroughs and these playthroughs are quick. We were finishing missions in ten to fifteen minutes. Subsequent playthroughs quickly led to predictable enemy patterns, which diminished some of the enjoyment. It would have been nice to throw an element of randomness into the mix, but that would have made the leaderboards invalid. I wonder if EA would have been better served by ditching leaderboards and offering a varying experience.
Players will be able to bring their unlocks into battle, but points earned will not go toward further unlocks, preventing griefing. It would be nice to figure out some way to have points count as it further reduces the replayability of Onslaught.
Regardless, co-op has come to Battlefield. It isn't perfect, but it'll be a nice starting point for the franchise when the content hits PlayStation 3 and Xbox 360 for an as-yet-unknown price on an as-yet-unknown date. As for Onslaught hitting PC, that's uncertain.