I've flown the P-39D a fair amount. It is quite a decent plane -- not a crappy plane as folks would think from reading historical information about it. It's fairly fast down low, turns OK, and handles and rolls well even at very high speeds. This latter point is useful against any planes whose controls get very stiff at high speeds (some of the Japanese planes, many of the 109's, for example), or that hit compressibility, or that come apart at high speeds (Ki-84's, for example).
No, you won't outturn a Spit in a stallfight -- but you can definitely hold your own in any type of multiplane fight, stallfights included.
Also, it's a rather small plane, so harder to hit.
So, you can turn hard in fights with most planes (except maybe Zeros, Hurris, Buffaloes, F4F's, FM2's -- but you are faster than those). You can use its good response at high speed against planes that are sluggish handlers at high speed (Zero, many 109's, Spit I).