The airplane stuff has been fantastic so far.
Flying - I imagine a lot of this is done with CGI. I'm just super impressed that they seem to have gotten all of the flight dynamics right. So often these things turn into video games, as real WW2 aircraft weren't maneuverable enough to be so-called "exciting" for modern viewers. These airplanes fly right for the era. In ep1, they even got the wheels-up landing correct, where the belly turret was ejected before landing. Superb.
On the ground, external - I am puzzled where the props end and where CGI begins. These things are spectacular. On the one hand, I'm totally convinced the bombers that these boys are crawling around and over are real, but on the other hand they're being treated far too roughly to be actual WW2 artifacts. The battle damage here is spectacular.
Internal - I know these are sets, but the sheet metal work on the aircraft interiors, for being reproductions, is just top-notch. There's simply no faking this. I'm totally convinced some set designer got blueprints from Boeing and just started bucking rivets. I wouldn't be surprised that these prop makers have created nearly flightworthy pieces.
I really, really want to see a "making of" reel when this is all done.