Insane levels of realism to keep any historian happy, but plenty of fun for the casual player.
Aces High II features an impressive spread of historical aircraft and vehicles in equally historical liveries; more of the latter can be downloaded for free from the game's homepage, adding customization to each player's aircraft. (In addition, with some effort you can make your own.) Each nation's military, even the obscure Italian air force, is represented here - you'll find British, German, American, Japanese, Soviet and Italian fighters, bombers, tanks, transports, and other land-and-sea devices.
The learning curve is quite interesting - you can get off the ground on your first or second sortie, you'll be flying high by the third, and within a few hours you'll be pulling your first rolls, loops and reverses. But it's also clear who the experts are: while getting your foot in the door is quite simple, getting to be on-par with the "aces" takes some concerted effort. But with some practice flying, even against the AI in the "offline practice" (also quite fun in its own right) soon you'll get up the nerve to find your first real dogfight.
My only complaint is the pricing for the arena modes; for avid FS fans, it's worth the ~15 USD a month; for people who only dabble in aviation, give yourself the trial period and maybe a little more before shelling out.
All in all, it's quite an experience; if you don't rely on graphics to make a game for you, and you think you're up for some hardcore realism and intense, fast-paced flying, this game is for you.