After playing the entire Security campaign (without "What Ifs"), I can tell you about how the gameplay has been so far, also I would like to note this is completely opinion and you can feel free to state your own opinions on what you thought about the game.
The AI in this game is very annoying, but that depends on how many of certain classes are in the match. The friendly AI is incredibly pathetic in the ways of using what abilities they have, and the lack of assistance in firefights. However I can overlook that, if it wasn't for the competitive enemy AI which have great aim, and keep the gameplay interesting, engineers will build barricades, operatives will hack your turrets, and soldiers will blow your hard work up. But, does that mean that the AI is good? Sadly I say no, the intelligence of friendly AI has made me fail a certain mission twice or so, and thats not exactly fun... however, I liked seeing the failure cutscenes.
The guns are fairly balanced to what I've seen, however I feel the shotgun when used with the enemy AI is just punishment to those who don't have a nearby medic. I've been using a light rifle and a SMG and I think the accuracy on both hip shooting, and ADS were fair. I didn't use many different guns, but I gotta say ARs were not very good in my hands.
I had been hoping that me and my friend would appear in cutscenes TOGETHER, however that is not the case we share the same role in cutscenes as a silent soldier always checking his gun, or looking for enemies through his gun sights.
Another noticeable problem I've seen is the massive difficulty jump from 1 player to 2 players challenge (Be more objective), when in single player the AI can be countered with using cover and being SMART (pun intended, also a bit of a hint), but when I played the same challenge 2 player on the same 1 star run, the AI has shown a huge increase in accuracy, and even camped a objective location mercilessly.
But, I should probably get onto the positives, the game is beautifully pulled off, and the controls are rather fun to just jump around and find new areas to flank from. The game's balance of classes are well done too, the classes feel great and the basic kits are all thats necessary in being a good soldier/medic/engineer/operative. The rest of the abilities are just for your playstyle, which are also done well, I still haven't reached a high enough level to get most abilities but the stuff I use now, works for me. However, there are drawbacks, like for the 3rd person ability you can't move and I didn't know that which caused me to get myself killed.
The challenges (besides what I said about the "Be more objective" challenge) are fun, and can help you in being a better runner, defender, attacker, and escorter (If thats a word). I can't really say much more about challenges more then its just a fun little way to practice your skills.
Lastly, I can say that the game so far has be enjoyable and I prefer being a defender then a offender, when playing the defensive missions, I felt less anger towards my friendly AI, but I can see the AI getting fixed hopefully in a future update.
Oh and by the way, I know I say alot against the AI, but thats the only thing I really didn't like about Brink, and for all you people that want a number as a rating I guess I can see this game (so far) as a 8.5/10, the AI really bugged me, and I have yet to play the multiplayer.
See ya online! Tell me what you think about Brink.