I'm guessing this fella is about 30 cm across, and has a better mental model of the world than any creature with a smaller brain and brain/body ratio than a chimp.
We're truly fortunate that octopuses 1) aren't social creatures (generally), and 2) only live 2-3 years (7-8 years max for arctic species). My guess is that evolution made a far more efficient brain in their lineage, and if only it were accompanied by decent oxygenation (their blood is rather poor) and a long lifespan they would have made life miserable for our fish ancestors before they ever emerged from the sea.
Actually, the real limitation on octopus intelligence is that, like all cephalopods, they die after their first reproductive cycle. That means they don't get a chance to teach their offspring the way birds and mammals do.
If they could survive reproduction, yeah, we'd probably be fucked.