Only had the game for a few days, but wow.
Really only one in the past comes to mind and that's Warbirds for the Lynx system. Was a great game for it's time. Since then, I've owned about every handheld and never found a really good flight sim. This game seems to be what I was waiting for.
Only downside I can see right now is no multiplayer, I think that would have been great, even ad-hoc. But this, is really not related to game play in any way.
I admit I tried and failed over and over again on the first few missions, finally changed the difficulty a little to let me complete missions. You're allowed changing difficulties by allowing unlimited ammo, unlimited fuel (I continually ran out of fuel chasing enemy planes) and greater damage to enemies.
The game itself it laid out in a basic way. You have career mode, and that's it. Within Career mode you can choose any of the 3 campaigns offered, Ardennes, Battle of Britain, and Stalingrad. Each mission is available to you as well, so you can skip missions and play another if you're stuck on one.
The game is very arcadey, but it has to be given the limited controls with the DS system.
All in all, a great handheld game and worth the $30.