The game is from Gaijin Entertainment, who also developed a couple of other decent combat sims like Il-2 Sturmovik: Birds/Wings of Prey and Apache Air Assault, and that shows. There are tons of different aircrafts and variants of them from five nations available, all modeled in high detail. The fight model is great and can be highly realistic, depending on the selected level of realism you play on. And the scenery is detailed and gorgeous.
You can also play tanks, but I haven't touch that mode yet (due to a lack of interest). In the future, there will also be a naval combat mode. Up to the most recent patch, plane and tank combat was separated, but now there are also combined game modes.
There are a couple of different game modes. Online, there are arcade battles, realistic battles and simulator battles, which differ in the realism settings and the choice of available planes for the teams: in arcade mode, both teams can choose every plane, in the other modes, you are limited to planes fitting to the team's nationality. For example, when you play on a Battle of Britain map, one team can only choose German planes, the other only British ones.
There are also single player (semi-)dynamic campaigns and missions available, though you have to unlock the latter with in-game currency.
From the technical side, the Mac version does not really convince me. For one, it's not particularly well adapted to Mac specifics. For instance, it dumps a couple of folders right in your home directory, instead of putting them in more fitting sub-folders. It's also not very well optimised. The game defaulted to high settings on my 2012 Retina MBP at 1440x900, but struggled to achieve 30 fps. Mostly, it averaged at 15–20 fps. To get a half-way decent and fluid framerate (ca. 30–40 fps), I had to take down the settings to medium (which still look quite good) at 1280x800. Reducing the settings even more helps almost nothing: more than 50 fps seem to be impossible, and even then it reaches that only on occasion. (And the game really looks like butt on these settings, actually running in an upscaled very low resolution.) (EDIT: I found that the framerate also highly depends on the map. The values I've given all are valid for a map set in the relatively flat Southern English countryside. On the very mountainous "Cliffed Coast" map, the framerate is basically halved.)
There are also a couple of weird interface bugs. For instance, the game reverses the axis of my scrollwheel, also when using it for its intended purposes (i.e. scrolling), without any obvious way to change that. Which leads to my next criticism that the user interface is in part not very clear.
On the positive side, the game recognised almost all buttons and axes of my Saitek X52 Pro flightstick without problems, which is probably a first under OS X. (In other flightsims, including Falcon 4 AF and X-Plane 10, I had to use ControllerMate to make use of all knobs and levers on that thing.)
If you don't have a flightstick, you could also use a gamepad (which I haven't tried) or even keyboard and mouse. For the latter, there's a "mouse aim" mode that actually works quite well, but severely limits your ability to control your plane.
The biggest problem I see in this game is its free-to-play nature. While it's technically possible to unlock every (or at least most) things simply by playing, this will come down to a horrible grind. You gain all the game currencies for kills and completing missions, but in relation to the amount you need to unlock new skins, missions or even planes in pitiful amounts. If you want to get your desired plane faster (and the crappy starters plane will make you want that), you either play a lot of games, or cough up real money.
The bottom line is that I'm still a bit undecided about the game. On the one hand, it's a very decent combat flight simulator – and these are very rare on the Mac. On the other hand, it's tarnished by the not very good port (at least it's stable), some design choices that were made to cater to casual players and the free-to-play aspect.