The thing I haven't done due to the download pricing is back up very much data into B2. Since the B2 API doesn't support renames, you have to download and then re-upload a file to rename it. I would be storing a lot more there if renames were available! [Note: they may be available now; I haven't checked for a few months. Please tell me they are.]
I was an early adopter of B2 and uploaded a bunch of files with naming that's incompatible with the "virtual folders" of the web UI (wrote my own script to handle this since Duplicity et al did not yet have support). Now I'm stuck and the only solution offered is just "pay us to download everything again".
Also, the web interface does not have any pagination. Since virtual folders are based on naming, and since rename support is not available, this can flood the browser. I have a bucket that crashes the tab every time I try to open because it's not paged. I could write a little Greasemonkey injector to page it myself, but meh, I don't really care enough to do that. Instead, I just don't really use B2.
When I discussed these issues with support and suggested that at least pagination on the web interface was a basic thing that engineering should be able to accomplish quickly, they did not seem to agree.