I have had very good experience (four countries now). In two instances (Japan and India), I was initially not too thrilled with the speeds. I could turn off auto-join and find a provider I knew would have better speeds and could connect and continue without a problem. So, it appears one is not tied to a single provider.

The only problem I have faced is when I used an unlocked phone that was not set as from Verizon. The support were helpful, but it was a pain after restart again. I switched the profile to a phone provisioned as a Verizon phone and it has been flawless. This may not apply to you. I do a lot of testing and I change the phone's country code all the time to test signals and software. When I set the phone as a "global" unlocked phone, things break. But when the phone is set as a US Verizon phone, among the most fluent global roaming you will find.