I fly the P-39D a lot. The thing that makes the P-39 workable is that it's low-alt speed seems middle of the pack and not horrible and that it at least isn't the worst turner in the game for stallfights. Also, it handles well at high speeds and can get up to high speeds without hitting compressibility. The usual stallfighting favorites (Spit's, LA's, Nik's, F4U's, etc.) will all outturn it in stallfights; but it can hold its own in a stallfight vs. P-51's, and it can beat FW 190's in stallfighting. Also, if you've got some energy, you can turn with decent-turning planes for a while, until it degenerates into a stallfight on the deck. In speed down low, it's roughly in the speed range of Spits (a little slower than a non-wep Spit 16, for example).
I think it is a much better plane than the P-40, which surprised me, given what you read about the P-39 from a US perspective. The Russians seemed to love it -- and if the AH performance is like real life, I can see why.
Basically, the late-war planes are generally better planes, as you'd expect; but you can, with a little care in your flying, be competitive in the P-39.
It makes me wonder what the P-39's history would be like if it had a 2-stage supercharger or a turbocharger as, I think, was planned initially by its designers.
