I'm in the lower usage group. I just checked yesterday since I'm deciding what to do for a new plan. From Jan 2011-Aug 2012 there was only two times I broke the 550MB mark.
I'm with AT&T but out of contract at the moment. I will be getting the new iPhone but can't decide which plan to go with. I have the 3GB plan with a grandfathered $5/200 messages. I want to be able to tether and increase my text allowance but I don't need additional data. I use GV Mobile+ for when I'm getting close to my limit but I really don't like bouncing back and forth between native texts and GV.
There's the new entry level share plan for $85 that gives 1GB of data and allows FaceTime and unlimited texts. It seems good but with tethering I'd be concerned it may not be enough. The 4GB share plan is more than I want to spend but would definitely over all the bases. There's still the 5GB with tethering which doesn't allow FaceTime (not a deal breaker) and texts would remain the same.
I don't stream movies or TV shows. If I'm going to watch something I load it onto my iPhone or iPad ahead of time. Most of the time I update apps from my computer and sync so that's not a concern. Websites, occasional music streaming, emailing photos and YouTube videos and games that sync and save data through iCloud make up the majority of my data use.
Verizon offers a 2GB share plan but if I'm not on LTE there's no simultaneous data/voice and their 3G speeds, in my area, are not that good compared to AT&T.
At least I have a few weeks to figure it out.