I own BOS and its extension BOM, with the extra purchased aircraft. It has "wow" graphics, cool shadows, etc. It has been improved over time to be actually worth playing.
However, it is like wearing a straight jacket. It is so limited in scope, number of planes, places, etc. Though I enjoy the flying, I feel entrapped into a very limited environment where I have to keep paying more and more to expand a little here, there, etc. Great business model, even Combat Mission went that route, but its just so, well, disappointing.
When you get down to it, even though graphics and effects are worthy, it is the flying and combat that is the most important. BAT has that and an immense variety. It still delivers the realistic flight modeling I want, and it does 1,550+ times, all over the world, all through flight history. After flying sessions with BAT, BOS just feels too restrictive.
So even with the better plane and land graphics, better shadows, more 'wow' looks, (horrible campaign), I've had little time for BOS. Even when I'm there, it doesn't take long for me to lose interest and be pulled back to the world of BAT. Sorry BOS, BAT is where its at.
Thanks to all the modders, the old IL-2 just keeps trucking and constantly getting a new set of wheels. It might look a bit old now, but its still flying, and due to the modding community, pretty damn well.