I don't have any experience with these but, have always thought it's an interesting idea. You can buy the containers cheap, and since they aren't very big, I would think they can be finished out pretty cheap as well. I would say the biggest factor is finding an area that could support a STR of that nature. Popular outdoorsy type vacation areas would be big hits. (Lakes, outside of natural parks, etc.)
I was going to do my off-grid home with them, but transport to my location (desert) was a problem. They are not as cheap anymore as you would think, and they need a LOT of work - cutting for the windows/doors is a pain, the interior wood door has been sprayed with deadly chemicals, etc. I still think they have a great possibilities as off-grid homes because the basic structure is already done. However they are small and you need to put a couple together to get a decent size. My observation is that most people do not want to stay in a house that is 8 ft wide.....you'd need (2) 8' x 20' at least IMHO....