Quick summary of my experience for the past year:
Have had a tmo line just for the roaming/ streaming netflix on the go in the states for the past year.
Has worked great for me in the UK, Germany, Italy, Netherlands, Sweden, Poland, Trinidad, and France. Each time I had reasonable speed- could stream music in ideal conditions, but it was sufficient enough for emails, reading articles, etc. It was good enough I could use at least 1.5GBs a week in data.
Haven't bothered to try anything like a vonage app, since I just pony up the 20 cents for the good connection.

Data was almost unusable for me in Mexico, I think it was TelCel.

In Argentina I got lucky and Movistar was unthrottled, had a full LTE signal. I would just have to manually select the network.

It's seamless every time I enter a country, do not have to switch sim cards or mess around registering. For anyone doing a bunch of country hopping in a short period of time this plan is great.