If it's really free and remains so, it'll be cool. Oddly enough, I'm in a tiny little niche market where there are a few folks who've had really nasty experiences with eBay, and hence refuse to do business through PayPal. The accepted alternative right now is they send me a check, but this thing looks like a good alternative.
This will be very handy indeed for a number of circumstances, for the low, low price of free.
Trust is a big issue though, people know paypal (mostly from ebay), merchants and techies know and generally trust square so getting that hurdle cleared will be a big deal.
I'll use it though for sure. An overseas option for cheaper than my bank (around $30 once everyone has had their slice), competitive with Xoom and I can see a lot of expats using this.