I have a Thunderbolt (4G phone), and I spent 5 days in Florida earlier this month. I have all of my photos synchronizing to both Google+ (reduced quality) and Dropbox (full quality), and I took a lot of photos. No WiFi the entire trip, but pretty good 4G coverage (except it would drop from time to time).
I left it uploading one night, not realizing that it was trying to upload a 5-minute 720p video I had taken, and failing for some reason - probably just too big. Woke up the next morning and my phone was hot to the touch, and had burned through 1+GB overnight trying to upload that damned video.
Normally, I struggle to break 1GB of usage. Last month, I sailed past 4GB easily (I take a lot of pictures, what can I say, the camera on the Thunderbolt is awesome), and if I had been in Florida the whole month I'd have finally had a use for this unlimited plan.