In the early stages of US involvement in the war, the P-40 was arguably America's front-line fighter. Considered by many to be obsolete, or at least obsolescent with respect to British and Axis fighters, the P-40 received a lot of bad press. The effectiveness of the AVG "Flying Tigers" was considered almost miraculous, considering the supposed inferiority of their equipment (and their always-precarious supply conditions.) In hindsight, the bad reputation probably was unjustified.
True, it was not as nimble as the Zero (but who was?), nor was it an agile dogfighter to match the Spitfire or Me-109. Allison-powered (the Merlin-powered P-40F and P-40L didn't become a factor until the Mustang and Thunderbolt pretty much overshadowed everything else), it lacked the high-altitude performance of some of its contemporaries. But in spite of all that, when the RAF found the appropriate niche for the Tomahawk, it became a star performer. Its detractors, of course, were unwilling to consider that they might have been overly critical.

… The high-altitude plane is at as much a disadvantage at medium and low altitudes as the low-altitude craft is when it ventures above its optimum level. At 20,000 feet the newest version of the Curtiss P-40 series, the P-40F Warhawk, is considerably faster than the FW-190 and as fast as the latest Spitfire V.
—Editor's note in Air News Yearbook for 1942.

Especially in the earlier phases of US involvement, combat reports had a lot of accounts of P-40 actions, and many of them reflected well on the airplane.

For convenience (mine), this page will also be the repository for links to earlier Curtiss fighters — the famous Hawk family.