I'll start out his review by stating something that may be a bit surprising given I am giving the game an average review (3/5 stars). I really like Brink. I had my doubts after seeing some of the quick looks from the X-Box 360 version, but the PC version seems to not suffer from all of the technical hiccups (which it appears the console version have received a patch for some of these issues) though still has some of the shortcomings in that the game just doesn't feel "complete". It's hard to give the game an outstanding score, but I definitely recommend people look into it more if they have any interest.
The single player is something that I won't delve too much into because quite frankly it's rather worthless. Splash Damage may have done more harm to themselves than good by including a single-player "campaign". The bots that you play with are relatively horrible, and although the cut scenes before each mission have solid voice acting and appear very well done, they are short and become repetitive as they really don't tell you that much. The audio logs that you unlock as you progress through the game help to fill in the back story and help you understand the world of Brink a bit more, but it really isn't that important and I think most of us are just going to focus solely on the multi-player.
Abilities and weapons are unlocked in a fairly quick manner by getting experience from playing any of the game modes and completing the games Challenges. The challenges include from a quick run-down of the objective based game play through increasing difficulties, escort objectives, a sort of tower defense style trial that requires you to use turrets and mines as an engineer to hold off several waves of enemies, and the least difficult but most fun one which is centered on teaching you the game's Parkour/SMART system. Depending on your skill level, these challenges can be completed anywhere from around 45 minutes to two hours, they give you a taste of the game play and help you understand what things will be like once you jump into a multiplayer match.
The challenges help a bit in understanding Brink if you are new to the Enemy Territory style of games. I can't comment on how effective these and the tutorial videos included with the game are, as I have experience from the previous games and wasn't really coming to Brink "fresh" of how the objectives work.
Classes seem to do what you would expect them to do, the game forces teamwork in a lot of ways with the use of individual buffs that you can apply to yourself and others. Medics apply a health boost and can revive fallen teammates, Soldiers can resupply ammo and blow up objectives requiring explosives, Engineers can plant turrets, mines, and repair objectives, and Infiltrators can disguise themselves as enemy units and hack objectives.
Although there are some utilities to help out players new to this type of game, I do think the game lacks in really giving you a great idea of what you should be doing at any given time. The objectives are great once you understand how to select one and have an understanding of how the game works, but there is no overview of the maps that explains the objective clearly or shows you the layout of the map beforehand. I think that this isn't a problem for someone familiar with Objective based games like ET:Quake Wars or Wolfenstien:ET, but may be a hurdle for some.
Multi-player is a blast to play, there are some concerning choke points on the maps that make it extremely difficult for one team to finish an objective but I think that this may be easier to overcome once more people have unlocked all of the abilities and are more familiar with the maps. If you enjoyed the Enemy Territory games you will likely enjoy this, it is smaller scale than Quake Wars but that isn't really a bad thing - it seems this game will end up being more 5v5 when it comes to competition. I know I watched a live stream of a match just last night put on by TheBrink.tv (The same people behind QuakeLive.TV) and although the observer tools need some work I still enjoyed watching the stream while I finished my night at work.
Due to the lack of any substantial single player content, the game is not worth the full $50/$60 - I would say if you can get the game at a discount it is well worth 40$ or so. I believe at the time of this writing there is a deal on Newegg and probably other sites for about that price.
Brink is a fun game that will appeal to those that were fans of Splash Damage's previous games, the multi-player component of the game is great and the Challenges are a nice way to get a feel for the game before jumping in to the full thing. If you are looking for an team-centric multi-player shooter than I think Brink will be great for you, I certainly enjoy it so far but I do recommend looking for a deal as the game doesn't quite live up to a full fifty or sixty dollar price tag.