The M-8 is fabulous. I just discovered it for myself a few weeks ago, and I am in love.
I've used M-8s to defend against M3s (and more) with great success. I'll take HE rounds, ramp up to 60mph towards where the bad guys are coming from (heck, at 60mph, I can afford to sneak around behind them) and when I'm close enough, I lay a couple HE rounds next to the M3s, causing them to tumble or explode.
I like using them against tanks w/ AP too. I'll get up to warp speed across the plains and jam over to behind the enemy spawn point or any inbound enemy tanks. I'll pull up behind them and start firing on them w/ my little 37mm AP gun. After a couple shots, I usually get the engines or tracks (sometimes the turret) and can either book out of there or finish them off. With the new turret tracking (eg, no instant turning), I can literally drive circles around a panzer or tiger, firing into them. The trick is to attack someone who will single-mindedly come after me with his main gun. Once in a while, though, I'll run into someone smart who gives up w/ the main gun and switches to whatever they have as their machine gun up top.
The M-8 is like a Corvette or Destroyer onland. Not a lot of punch, but more then enough speed to make up for it.