I did have a 20gb allowance with EE and was surprised two months running that I got text's saying I'd used 20% of this... which I realised coincided with my taking my MacBook to work and tethering. Damn, that thing burns through data even when I don't think I'm doing anything! For £5 more I stepped up to a 100gb allowance (now £25).
Two months ago I used 29gb, last month 9gb and this month 7gb half way in. Considering how cheap it is, I'm happy to have a 100gb available, but doubt I will ever need more than half of that.
For me it's not that I try and always be on WiFi because in reality the speed difference doesn't really show unless you're downloading a movie or something. For me it's more about I have a PC and a nice TV, so I have no need to stream things to my phone. I don't like/need to watch stuff while out and about.
The thread is a little pointless though, as everybody's needs and uses are so different...