The P-39 was, quite rightly, replaced as soon as possible in the Western Allied air forces. And it was not a performer for the ETO, but there are reasons for this.
When the XP-39 was rolled out on to the pan and test flown, it was quickly understood as being one of the best fighters in the world for it's day. Heavily armed, extremely fast and quick to altitude. But this centered around it's turbo-supercharger which, to cut a long story short, was deleted because the inlet caused too much drag. A massive mistake, amongst the others such as the increase in fuselage size, reduction in wing-span and reduction in cockpit size. Bell couldn't stop these changes, being in debt and having no political strength to stop anyone doing anything.
The P-39 itself was a poor performer at altitude, which leads to it's awful reputation in the ETO where high altitude combat was the norm. The RAF dispised the P-39 and many pilots refused to fly it. On the other hand, some RAF pilots did comment that the P-39 could match a Bf-109E in a dogfight below 10,000 feet. And this is what leads to the success of the P-39 in VVS hands; on the Eastern Front combat generally took place below 10,000 feet, in fact most of the time it was below 6,000 feet all the way down to tree top height and this is where the P-39 was in it's element. It was a capable plane at these heights but I would much prefer to be in a Bf-109F or Fw-190A.
Basically, for the ETO it wasn't suited.
The P-39 served with the RAF, VVS, Free French and USAAF in the PTO, ETO, MTO and Eastern Front. It's hardly a record of a "crud" fighter.