5GB seems like a reasonable limit for some people to hit (if they have multiple iOS devices to back up), but 25GB or more? Nobody seems to be able to explain how they can fill that.
I could use more than 5gb for backup, but I'd they allowed true file sharing (like what they killed with iDisk) I would def use that 25.
My dropbox is 9gb and I dumped a lot of files to my harddrive to get it that low--so as to not pay.
I do a lot of observation and interview research, and would be able to use that space to backup the apps I use for that (I don't keep that stuff in iTunes).
So right now i could easily use 15gb, and would be able to fill the rest in a couple years. Just one example, but there you go.