Someone has already stated that they're not easy to keep, so I'm not going to get into it.
I've kept 3...all Bimac Octos and once you get the hang of it, they're not that hard, at all.
Bimacs are also the most common. Fully grown, you can get away with a 55gal. tank, for them.
The only downside, which was again, already stated...their 12 - 14 months lifespan.