Mao's Red Army kept the Japanese on it's heels, when they chose to fight. Oftentimes, they didn't choose to engage, as they had little equipment and no mechanization at all. If nothing else, they kept some of the Japanese occupied with chasing them, which is something, in itself.
Stillwell, Chennault, and others worked well with Chiang's troops, but there was a lot of corruption there, and it was hard to get the Nationalists to fight.