While a revision of Rowan's Battle of Britain from 2001 the game is as fresh as anything out today.
The graphics and flight models are superb, the AI is the best in the business and the game offers a hard to describe immersion factor that no other sim can achive. The feeling of flying in your plane toward a massive 200+ bomber formation over London just has to be seen to be believed (several clips are avilable on youtube).
Many people compare this game to the Sturmovik/Forgotten Battles/Pacific Fighters series and while the later has a more detailed damage modeling (although to over-the-top to be realistic) and larger plane selection, everything else is significantly better in BoB2 (especialy the AI).
But beware: the stock game is buggy, so download the latest patch from the developer group, who still actively improve this work of art (the latest version while writing this is 2.07).
The good: great artificial inteligence, flight models and graphics, insane immersion effect, best Battle of Britain experience, unparaleled developer support
The bad: stock game is buggy, ground detail and damage visualisation could be better