I agree with the mifi thing, but if your own phone will hotspot or if you have an extra phone to use as a hotspot, I would look into a SIM for that over renting a mifi. I got a SIM in Lisbon and then again in Amsterdam, and the Amsterdam one was good for in-Europe roaming.
But buying a mifi, so that you don't have to worry about sending it back, is different. There's another thread here:
http://www.flyertalk.com/forum/trave...fi-device.html
This thing is great if it gives coverage and speed where the user is going to be. A 5 GB data load costs $15 and doesn't expire for 2 years. I wish the device was cheaper, but it's a better value than a mifi daily rental. Also, there's a review on the other thread, and the reviewer got it at HKIA for half the price listed for online purchases. I used tep twice in 2013, and it was the opening of a whole new world. But those two uses including shipping fees cost more than the pokefi thing with 5 GB of data included would cost today. I've been recommending it to all my friends, while making sure to tell them that they should read reviews about speed and coverage. I haven't found a cheaper solution for full-speed roaming, provided that the performance matches the claims.
Oh, one more thing about t-mobile is that they introduced something called DIGITS. It allows you to do calling and SMS over the web on a computer, or on any device that will support the DIGITS app. (Android 5.0 or higher, and whatever level of iphone operating system is required). So a t-mobile user doesn't have to have a phone that is specifically configured for wifi calling; they don't even have to have any phone at all.