I used them for when I went to Canada a few years ago, It worked fine here in the USA, and worked in Canada with extra cost of international added to my plan.
I did this as it was the cheapest way to have a phone in Canada that I wasn't going to use much. Far cheaper than getting a sim in Canada.
Their support was great, one time I spent my $10 international extra I had put on, so my phone calls went to them instead when I dialed, so I was able to add another $10 over the phone with the support person.
I've been thinking of switching full time to mint, as I currently don't have data with my T-mobile legacy plan, and occasionally it would be handy.
You get free calling from USA to Canada and Mexico included which was nice for me prior to my trip.
Usually you can do the 3 month and then switch to a yearly payment, plus they have a 7 day money back guarantee.
I didn't port in my number as it was just temporary at the time.