If you want to see the success of Square, go look at any new food truck or coffee shop or streetside vendor. It seems to be the new solution for that sort of thing.
Portable chip based card readers have been around longer than Square. Hell, I remember using them (pre-chip of course) since around 2000ish. And today they are just ubiquitous. They are straightforward and user-friendly for the user and today the Banks almost give them aways with your merchant account. I'm still trying to understand what problem Square is trying to solve -- other than wanting to use your iPhone for everything. I just don't see a viable business with them.
What you have is an imprint machine, where the merchant sticks your card over a paper with a carbon copy, slides over the top to make an imprint, then hands the piece of paper back to you for signature, then rips out the top page for himself while handing you the carbon copy one.
As you can imagine, this does not go well in food truck environment, where the mode of operation at crunch time might be "process as many customers per minute as you can".
Although to be fair it does end on a non-fatalist note that leadership may be able to tweak the business into something less grandiose and more oriented towards grinding out a profit on a better-than-average commodity product.
Oh by the way, Square's market is limited. The USA still uses primarily the unsafe magnetic stripe for payments, and even Canada is phasing out its use (in Interac at least)
http://www.forbes.com/sites/tomgroenfeldt/2014/01/28/america...
So I bet you'll see very quick adoption, within a couple of years max.
If Square can't even meaningfully make inroads into Canada, I highly doubt they will be able to make inroads into anywhere else. But, we'll see...