We've been in London again with our XM iPhones and mobile data has been much worse than previous recent visits.
I spoke with customer service and they didn't offer any suggestions beyond a network settings reset which would wipe all our wifi passwords. Didn't want to do that.
What they DIDN'T suggest was turning off AUTO for cellular network selection ("Auto" probably selects the strongest CELL signal) and choosing an alternate carrier. The auto choice was "O2 - UK" and accessing data is horrible. I switched to 3 UK and got an immediate improvement. Still experimenting.
I just thought I'd post here in case someone is looking for a solution to a poor data connection while roaming internationally.
Depends on the network wherever you are, really. I was in the Bahamas and the best network I could find was a 3G provider which wasn't bad. I had my device on airplane mode most of the time connected to wiFi.
Are you using an iPhone? In my android device, I can manually select a network operator and network mode so when I go overseas, I can choose which one I connect to.