the first time i flew the 39Q, it absolutely surprised me. it surprised me even more when i got rid of the gondola guns.
The P-39 (both Q and D) is quite a decent plane below about 13k. Turns decently, rolls decently, has decent speed down low, is excellent in high-speed dives. I've flown it a lot in the MA.
It would be interesting to know how it would have been regarded if it had from the start gotten a good turbosupercharger like the P-38, which was the original plan.
It is also interesting to know that the Soviets used it as an air-superiority fighter (i.e., against 109's, 190's, Ju 88's, Ju 87's, etc., not primarly as a ground-attack plane as often mistakenly stated in literature), loved it, and did very well with it. Three of their top five aces flew the P-39. The difference is that a lot of the eastern-front fighting was at about 15k to the deck.