IL-2 is a fantastic flight simulator. While many flight simulators have their strengths and weaknesses, the IL-2 series became the pinnacle of the WW2 era flight simulator genre - it has everything: a fantastic flight model, great damage modelling, many (well-modelled) aircraft to fly, and good artificial intelligence, along with great graphics and sound. It's also very approachable on the difficulty level, due to a wide range of settings, to accomodate both the newbies and the hardcore flight sim fans. No other simulator comes close these days, when you consider all those parts; IL-2 is the ultimate package for any flight simulator fan.
For those who like single-player, there's plenty of options. There are pre-made missions, an assortment of (dynamic!) campaigns, and both a quick mission editor (for practing dogfights, for example) and a built-in mission editor which is very capable. The dynamic campaign engine is great, but make sure you update it with the latest version (look for further information on the game forum, I'll see if I put it there). If for some reason you don't like the missions generated by the built-in dynamic campaign engine, there are a few free third-party options available as well.
It really, really shines in multiplayer. You can fly missions competitively or cooperatively, in many different maps (in missions made with the mission editor, or even dynamic campaigns), and it works beautifully, even with some moderate lag - I could play well in some servers even with a 400-500ms ping. I've played this game in multiplayer more than any other game I own (and on a 56k modem at the time), and it has always been a blast, a real exhilarating experience.
All in all, this is one of the gems of the combat flight simulator genre. If you like flight sims, you probably have this game already. If you don't have it already, get it now. It doesn't get much better than this.