The four styles of proton pack emission available are: standard Blast Stream, Dark Matter Generator, Slime Blower, or Meson Collider. This provides a small semblance of tactical strength, if players choose wisely; of course, most just pick their favorite and blast away. Enemies will randomly drop power-ups that give you limited capacity to use the other weapon styles, and there's also other bonuses like a personal Etheral Shield, the Ghost Stunner, and a couple of other small timed perks that shake things up without really providing much of an impact on the gameplay.
Each proton pack style has a secondary fire mode: the Blast Stream doubles as a Capture Stream for wrangling and trapping weakened ghosts, unfortunately the Bosun Dart blast from single player is not available in co-op. The Dark Matter Generator's primary fire mode is a shotgun-like blast that works well against smaller ghosts that don't need to be trapped; its secondary mode is a Stasis Stream, which slows most ghosts down to a crawl. The Slime Blower is just that: a slime blower that can reverse the effects of a puddle of damaging black goo; but it also features the Slime Tether, which can be used to move objects, tether ghosts, and even trap them. Finally, you have the Meson Collider, which provides two types of single blasts of different strengths (and different effects on your pack's heat level). Choosing a primary weapon style grants you limited access to that feature at the start of each co-op round; aside from that, you're still going to use your Blast Stream a lot. Whichever weapon style you choose is the one that gets damage upgrades the more you use it.
Contrary to fears of gimped co-op maps, the multiplayer areas are not pulled directly from the single player campaign; each is reminiscent of its campaign counterpart, but completely original. Some have a variety of individual spaces in which to run amuck, like the high-rise office with its connected rooms. Some are built around a central spot, like Central Park West, centered around a crater in the intersection. In either single-player or co-op, your character cannot fall off of edges; while that makes things a little too linear, it also eliminates the fear of falling off of an edge while trapping a more difficult ghost. The maps are anything but bland, full of props and scenery that can be destroyed. No environment is left the way it was when the match started.
There is one glaring problem with the online multiplayer that local co-op would have helped to alleviate, and that is a sometimes nasty case of lag. While players and ghosts hold real-time positions without a hitch, ghosts that are lassoed often pop around, making it difficult to pitch in with your Capture Stream. The one competitive mode, Slime Dunk, is the most affected by this, because players can "steal" Slimers with the Capture Streams if you take too long to dunk 'em. Since there is no particular ghost that requires more than one Capture Stream in order to successfully trap them, this isn't as much of an issue in co-op...but it can be annoying, especially when you're trying to help trap one ghost while four more are still attacking. Wrestling a difficult ghost can become quite a chore when three people are slamming it all over and trying to pull it in three directions. In my opinion, traps should be one-time-use in co-op, so that each player must pick up their trap in order to throw a new one; as it stands, throwing your trap in a new location just makes the previous one disappear.
Because your team is split up at the end of a match, co-op campaigns are the best way to play with a group, unfortunately, playing a co-op campaign isn't exactly how it sounds. While we were under the impression that we would be subject to four progressive and streamlined campaigns, in actuality each is just a series of related maps with randomly generated Instant gameplay types. The only real benefits to playing co-op campaigns are that: #1 - you get to play straight through without returning to the options menu, and #2 - there are two Achievements/Trophies specific to co-op campaigns. Private game for you and your friends is to create an Unranked room, which does not allow you to keep your cash earned, and still boots everyone back to the menu when the match is over. It's a pain to re-invite everyone each time; Xbox Live users will be thankful for Live Party, but I'm afraid that the Playstation 3 version gets another short-ended stick in Ghostbusters.