I've been keeping bimac octopus for nearly four years, and I currently have two. While I agree with jmadden's basic sentiment that one should not lightly decide to keep an octopus, I have had a different experience with my bimacs.
I've had my Bimac since it was about the size of a quarter and now seven months later it is much larger with a mantel close to four inches. ... the bimac has a very short natural life span.
Based on my experience, I think that the growth rate (and life span) of a bimac octopus is greatly influenced by the amount of food it eats, and the water temperature it is kept in. In the wild, the average monthly water temp ranges between 57 in February to 68 in August. The bimac I caught on Nov 6 2010 with a 3/4"-1" mantle (head bag) now has a mantle only about 2.75-3", because I keep it at 63 degrees, and feed it very sparingly. about 26 months ago I brought home a bimac that I would guess was at least 6 months old already (3-3.5" mantle) and it is still alive. It's a male, which might be a factor, but it is at least 2.5 years old. Because it's easier, most people have kept bimacs at room temp without a chiller (73 degrees) and fed them every day, and/or as much as they wanted to eat, and under those conditions, they will grow fast, and probably last less than a year after capture, even if you get a small one (18 months tops). I think it's still good advice to design the tank with the understanding that your bimac will die before too long, but under proper conditions, I think you could expect a baby to live two years. That would of course require a chiller.
The investment required to take care of an Octo the right way and to flourish is intensive
Maybe, but maybe not. To do it right you need a tank that is at least 50 gallons (65-80 is much better), that is 60-65 degrees F, and you can't keep many other species with a bimac (no fish). You'll need a 1/5 to 1/3 hp chiller, pumps that don't add much heat (external), regular fluorescent lights, and a secure lid. I got my 1/4 hp jbj Arctica chiller used on Craigslist for about $300, and later (after patiently for looking for months) found another, as a back up, for $250 (1/5 hp jbj). I built a secure acrylic top, and I use regular (cheap) fluorescent lights. I think the cost and electricity for the chiller is not much more than the cost of lights for a reef tank would have been, so it's close to a wash for me in terms of money. My bimac eats defrosted pieces of frozen shrimp and scallop in such small quantities that it's essentially free, and biamcs are notoriously hearty and tolerant of nitrate, and surprise temperature changes (power outages). making the tank escape proof, and insulating it with Styrofoam sheets can be a pain, but I wouldn't call the overall investment "intensive". I wanted to save on electricity even more, so I built a 2nd pane of glass onto two sides of my tank, and insulated the other sides, and bottom, with sheets of Styrofoam. The 2nd pane was a pain, but not absolutely required, unless you live where the dew point is often above 63 degrees (rather humid). Other types of octopus are more easily found in the aquarium trade, and can handle temps above 75 degrees, and so are easier to find, and keep than bimacs are (although I think bimacs have the best personalities)
I would recommend setting up a tank that is reef ready, think long term, so after your octopus passes away you don't have to upgrade everything to make your tank ready for fish and inverts. TONMO's website has a wealth of information and can further explain in detail the ins and outs of owning and enjoying your octopus. Do lots of research, patience, knowledge, good foresight and planning go a long way.
I agree with all of that. I happen to live where I can catch my own bimacs, so I can get away with having a dedicated bimac tank for as long as I like, but if you plan to buy an octopus, you never know what species you'll get (no matter what the seller says) so it pays to have a tank that is large enough to handle most species, and can be useful for fish, reef, whatever down the road, if you get bored with a (mostly) octopus only tank down the road. Do lots of careful research and prep before you get an octopus, but it can be reasonably done.