I will endorse KnowRoaming. I started using their SIM (in a second phone) for my travels to Europe and Asia. I pay-as-I-go for roaming data, and have found the service works well at a reasonable price. I prefer using wifi (on my primary phone), but when I'm lost and need a connection, I don't think that I've every paid more than $20 for a week's use (and it's usually $10).
My main phone is the Freedom Mobile, on a legacy offer that covered Ontario only for $30 per month. When I go to the U.S., I turn on the Ready-to-Go, and then turn it off when I return.
The international roaming service on Freedom Mobile was much better, 3 or 4 years ago as Wind Mobile, than now. I first noticed that World Traveller didn't cover Ireland, then Finland, and then Austria. (UK and Germany, yes). Hence the switch to KnowRoaming, which works even in China. In the U.S., it's more convenient to maintain the primary Freedom phone number, so I don't use KnowRoaming there.
This approach really optimizes for international roaming, at the expense of local service. I live in downtown Toronto, and still pay for a landline, so service interruptions are actually okay for me. Before the LTE was enabled late last year, I noticed that my service inside concrete buildings wasn't good, while friends could use Bell or Rogers without difficulty.
I also have a free phone number on Fongo. When I'm travelling overseas and want to make a call back to Canada over wifi, sometimes Fongo works better than Google Hangout phone calls.