I kept one for about 6 months many years ago. They're fairly common in the rock pools along the coast here in Oz.
There are a few things to consider before going down this path...
1. You won't see them most of the time as they spend most of their life hiding in crevices.
2. When they are out of their home, you'll probably have to look pretty hard to find them as the rings are only really pronounced when they are agitated or distressed/defensive. The rest of the time they are more of a leopard skin pattern that blends into the rockwork with perhaps some faint small blue rings/dots. Depending on the species/base colouration/mood these base patterns can range from really drab to quite elegant though.
3. Whilst their primary native diet is crabs, molluscs, shellfish etc. they will also attempt to devour any of their slower fishy tank-mates - usually without success though (unless the prey happens to be snoozing).
4. Yes they are extremely toxic - but they are also essentially very passive. This doesn't detract from the fact that tank cleaning becomes a constant concern. If your hand is in the water scrubbing algae, , who knows if your sharing the space with a cephaloapod that woke up very grumpy?
In other words, you are only going to get the pretty blue rings to show if you agitate him/her enough, which is kinda unfair on the animal.
But it is pretty cool to watch them demolish a small live crab in seconds. It helps if you live coastal near rockpools so you can provide a proper diet for them!
Think hard about the pros & cons.
