Finally, let me share my experience with the chickens themselves, from my sample size of one breed.
We've got Isa Browns - Best decision we ever made. They're extremely friendly to the point they get in the way. The only times I've been pecked is in the odd moment my hand comes between them and a nice, green, crisp piece of lettuce. If you've got a family, they certainly aren't about to take small pieces out of your children while you're not looking.
But beyond that, they're prolific layers. On a rough estimate, I'd say they lay two out of every three days, and I've very rarely gone a long stretch without any eggs. It really is unbelievable the mass of lime and protein they push through their bodies on a weekly basis. And these aren't small eggs either. On these grounds, I think two chooks is more than enough for most people. In addition to cooking our own omelets, we also feed close family, friends, neighbours and colleagues.
If you're a traveler though, you might want to consider your holiday-care options before diving in. It isn't quite so bad in winter when they can last days without any specific attention - but in summer its critical they're checked on every day. Without water, chickens will quickly progress to rigor-mortis in the summer heat - and its not unlike a chicken to accidentally tip out all of said water on any given day. We're are gifted with a host of friends and family who love eggs, and help us out on many such occasions.
But most importantly - the cost. It takes me about five minutes daily to keep them watered and fed, I also clean the brown gold out of the nesting box - it keeps them clean, and sees it added to the compost heap in small excrements. They chew through about $20 of layer pellets in a two-month period; a diet which is supplemented with an variety of free kitchen and garden scraps. The litter in the whole coop is replaced every two-three months as needed, at a cost of about $20. And the spoils: 400+ fresh eggs a year. But there's more! If you're a gardener, chickens work a lot harder than just laying eggs. They're a big source of home-made fertiliser, and they're always keen to dig-over and weed your garden beds while you enjoy a beer in the shade.