The Madstone Rage effect can be caused either by random contact with monsters in the Madstone Crater quest, or by the Madstone Boots raid loot. The duration is 2 minutes, and it provides a +2 bonus to strength, a +4 bonus to constitution, a +4 Natural Armor bonus to AC, a 10% enhancement bonus to attack-speed, and increases your BAB to equal your character level. However, characters cannot cast spells while in this rage, which includes activating spells from items (except potions) as well as spell-like abilities such as from Bards or Monks as well.
There are three different ways to acquire Madstone Rage: Through the Madstone Boots as a clicky, from the random spells cast while exploring the Madstone Crater, and from special Madstone Reaction effect of the boots. The clicky effect and the spell cast on you in the Madstone Crater quest are the same spell - as such they do not stack, but will refresh the duration to 2 minutes. The Madstone Reaction effect stacks with the other for strength and constitution, but not for other effects (allowing possibly +4 STR and +8 CON).
- Bug: The attack-speed bonus is is less than the listed as 20% as of Update 5.