I think that the P-39 is a good airplane below about 13k.
It rolls decently, turns decently, handles well at high speeds, and is decently fast. It's not the best at any of these traits, but its mix of abilities means you can be competitive in it.
The Soviets liked the P-39 very much for fighter combat, and 3 or 4 of their top 5 aces flew the P-39.