I've killed Panzer IV's, T-34's, M4's, and even Panthers with the M8 before. Its not completely hopeless, you just a lot of skill, and even more luck. That said, I don't take the M8 out except for when I'm just dicking around, trying to see what shenanigans I can perform.... such as trolling the hell out of inept Panther drivers.
However, I will say its cannon makes about the most effective ad-hoc AA gun in the game. Flat trajectory, high rate of fire, and the element of suprise!