In addition to their defensive bonuses (equal to torva boots), they provide 2% reduction in damage from creatures in and around TzHaar City. However, they are only useful when fighting TzHaar and TokHaar -- outside that area, and in PvP, the obsidian armour will triple damage towards you.
When used with the rest of the obsidian armour and either an melee, ranged or magic helmet, it will increase your accuracy by 10%, very similar to the Void Knight equipment.
The Obsidian boots degrade after 2800 hits, similar to the Barrows equipment. Once they fully degrade they cannot be reused, and new boots must be smithed.