The horrible launch for Brink kinda threw it under the bus, but after AoC came out the bulk of peoples issues with it were fixed - multiplayer servers being abysmally laggy and sub-par AI on even the harder difficulties.
It requires a lot of teamplay compared to other FPS' due to the fact that each 'mission' can require you to go through several different hoops as attackers and defenders to complete the mission. Rather than just planting a bomb and winning you might need to plant that bomb, extract a hostage then stop the enemy from destroying a bridge, all within the time-frame given.
The AI is still kinda wonky, even after AoC on lower difficulties. They do work together, and don't suicide off bridges anymore, but sometimes they focus on minor objectives rather than the primary for to long. When they do this it can cause you to stalemate into a loss. While it is annoying, generally one or two will follow you on defence if you set yourself to the primary objective manually (and are the appropriate class to do the objective), which helps relieve some of the silliness going on when they try to retake every command post over and over instead of just winning.
Characters are pretty damn customisable, unlocking new outfits and gun parts by leveling up, completing mission and beating challenges. Though character models are all vanity, when you get to changing the looks and attachments to your weapons you'll find the more gameplay changing bits in that you can make a gun faster, have more stopping power or accuracy, or add a giant shield onto it so you can frontline.
Certainly still a fun game to play solo, co-op or with a larger group of friends - as the campaign is multiplayer friendly (including playing as opposing factions) or by setting up custom games. Just be aware that if you're solo the AI sometimes goes a little bonkers for side objectives and they need a little push via command wheel to get back on track.