Depends on the Theatre of conflict.
True, but I think it was used everywhere?
The British had the P-400, but didn't like them so rejected them, and we used those P-400's in the Pacific along side other variants. I'm not sure if it ever saw combat with the Brits?
The Russians used it a lot against the Germans, but they generally flew low so the lack of performance at hight alt didn't bother them, and they like big guns.
We used it primarily in the Pacific against Zeros a lot, it worked, but wasn't the best.
So all in all I would think it's a below average fighter, but not the "worst"...
I mean, if you got dumped into a WW2 air battle, which would you prefer?
The P-39D-2 or the Bolton-Paul Defiant or the Farey Battle?