I've played WWII Online and still have a subscription (I like the infantry, ship, and armor aspects). For flying aircraft, though, I greatly prefer Aces High. They are very different games.
WWIIOL is half-scale Europe as the map, early-war. It is intended to encompass all forms of WWII activity -- infantry, vehicles, mobile guns, trucks, tanks, artillery, ships, and aircraft -- and geared toward the whole war effort, including working on supply of bases and building rank by succeeding in missions.
You don't get to fly what you want. At first, you don't even get to fly bombers or any of the better fighters until you succeed in enough missions in the lower-end couple of fighters (Hurri I and Curtis Hawk on the allied side). You might not even be able to fly anything if there aren't any missions posted with aircraft available or if all the aircraft have been used up.
If you do fly, in my experience, it is pretty sparse fighting, typically at altitudes even lower than in Aces High. They have nice clouds and landscapes. The flight model is decent.
The bad points for me are as follows. First, visibility just blows in WWIIOL. With clouds, haze (even the little that is there on clear days), inability to see your 6 position, inability to move your head location, the way they do identity of enemy aircraft (range at which you can ID and the way you lose ID and marking on the aircraft if you chance views even if in order to keep the enemy in view), I have a very, very hard time keeping things in view even in close-in fighting. I hated that aspect. Second, when you start out, you can't fly anything but the slowest fighters (Hurri and Hawk). Since it's impossible to see behind you, being the slowest guy out there is a huge handicap. You can't even fly bombers until you go up in rank. So, your early flight experiences are a lot of just trying to see behind you and evading lots of attacks of faster aircraft running you down. Third, there aren't a lot of planes around. I think the biggest fight I got in involved about 5 friendlies and 5 enemies. That happend once in about 20-40 hours of on-line flying for me. The rest of the time was 1-2, 2-1, 1-1 action.
Now before the WWIIOL people jump on me about the above, saying "But that's all more realistic," let me say that my dislike of it isn't that it's more realistic, it's that they pull of the realistic aspect poorly, making it less realistic. First, I don't mind bad visibility if that is realistic. But it's just not that hard to keep a plane in view in a close-in dogfight in reasonably clear weather in real aircraft in my experience. Their flaw is in the way the aircraft ID disappears if you view shift, even if it is to keep the plane in view and in that you can't move your head at all even to get some better glimpse behind you -- maybe not dead 6, but a little. If they want to be that realistic, then they should let you move your head, loosen your harness, have a rear-view mirror, or all of those things. Second, it sucks not getting to fly anything but the slowest fighters for a long time at the start -- not even any divebombers or level bombers or having rockets or anything that would make you useful in hitting stuff on the ground in a game that is (rightly) much more populated on the ground. Third, well, compared to the skies of Aces High, it's sort of boring in the WWIIOL skies. Yes, that is more realistic.
If you want more action and the ability to fly what you want, partipate in ground attack, base capture, etc., Aces High's main arena gives you that and WWIIOL doesn't. If you want lots of realism (and still exciting action), Aces High scenarios give you a much more realistic and involved environment than WWIIOL ever will.
Oh, one other thing I forgot that, for me, is very important. Under fairly common circumstances, frame rate in aircraft in WWIIOL can go to almost zero. This perhaps doesn't happen to everyone -- it might be setup specific. But I invested hours to figure out how to optimize my system, including getting help from several WWIIOL regulars, and my system is quite reasonable: 2.4 GHz Pentium 4, 512 MB RAM, and a Radeon 9800 Pro. Basically, if I fly into an area where there are even a small-to-moderate number of people on the ground at the same time there are some enemies in the air, my frame rate can go to 5 fps (sometimes less) for, say, 10-20 seconds while the code does whatever it is working on (loading more textures, frame shifting, who knows). Before I got help on things, it was unplayable -- I'd get into combat, and frame rate would go to zero. I'd crash into the ground without any aircraft control. These days, it is at least usually playable and OK, but far inferior to the frame rate of Aces High.
