While they have their uses, they're not all that easy to use for much more than storage.
First off, for a workshop type of place, they're rather long and thin. My shop- at least the machine room- is fairly long and thinnish, but not to the extent a cargo container is. They're not unworkable, of course, but it does have to be worked around.
Second, they're basically just steel boxes. Unless you get a reefer (refrigerated unit) they're uninsulated- meaning you freeze in the winter and swelter in the summer. That's even worse on the machines- yeah, I shoot everything through my own viewpoint - because even if it's not high summer or deep winter, the temperature changes between daytime and nighttime lead to condensation and rusting.
Point in fact, that's why I got that Nichols horizontal mill. The guy that bought it from the States, had it shipped up, and wound up storing it in a connex 'til he could get around to rebuilding it. After a couple of years, despite the door seals, everything in the container- including the mill- was starting to rust badly, so he decided it'd be better to sell it as-is, than let it get completely ruined.
So in order to use it as any kind of work space (let alone living space) you'd need to insulate it, which means both insulation and the sheeting or wall construction to cover it. It's not much of a savings over building a building if you have to buy the connex then build half a building over it.
I have considered, at times, buying one for just cold storage. If you keep it vented, like a barn, and have overhanging eaves, it makes a great storage box. People use 'em all over the place up here.
There was also one setup across the street from ShopGuy(tm)'s old shop- somebody had stacked four, in two seperate columns, thrown a roof over the top, a wall at one end, and a sliding barn-type door at the other. So he had four 40' vans worth of room (though I don't know how the upper ones were accessed- might have just been with a forklift) plus the space in the middle, which was roughly equivalent volume again.
I've always thought that was a good idea; you'd have four individual "rooms" plus the main work floor. Use the central area for the primary work (cars, building trailers, whatever) and use the bottom two "rooms" as dirty and clean areas: all the machine tools in one room, semi-isolated from dust and grit from the main workshop, and on the other side have all the welders, grinders, abrasive saws, sandblasters and plasma cutters, with fans and filters so all the major dust-and-smoke producers were also mostly isolated.
Then use one upper room for an office or offices (or even a mini-apartment depending on how wild you want to get with things like plumbing) and the other for storage.
Problem is, what with insulation and foundations, trusses and all that, using the connexes really wouldn't save all that much over just building a standard stick-frame shop.