Mega Medicham has Huge Power. So assuming you max out it's pretty average ATK stat to 328, you have one hell of a monster. Huge Power doubles the attack stat bring it to a maximum of 656 (highest attack stat in the game par mega mawile). This is higher than most Pokemon with Choice Band. Pair this with a pretty sweet 100 speed and deadly base 130 power stab in Hi Jump Kick, you have one hell of a sweeper.
Its defiantly a good Pokemon, no doubt about it. To put it into perspective, in reality it actually has a base ATK of around 300 and a base speed of 100 which is far from bad.