I almost went with them recently, but decided against it due to the roaming data costs. Whilst the calls when roaming are quiet cheap as you've said, the roaming data rates start at about 10 cents/meg, which is $100/gig.

I ended up going with Google Fi. Domestically it's more expensive ($20 + $10/gigs compared to Comcast at $0 + $12/gig - although Comcast charge in whole gig increments, whilst Google do partial), but with enough international it'll work out cheaper for me. Plus T-mobile coverage is a little better where I live than Verizon.