You might as well register as its free, and if you are doing something that requires all your bandwidth it will throttle the BT Fon right back. Its a bit of give and take. I’ve only connected to BT Fon a couple of times whilst out and about, but the hub also broadcasts an Openzone hotspot and this I’ve had much more success with on other peoples hubs.
One major downfall seems to be that after enabling BT Fon, the hub doesn’t actually broadcast anything. Each time I’ve had a new hub I’ve had to do a manual firmware update as even after a couple of weeks its still not broadcasting.
I sometimes connect my work laptop to the BT Fon channel on my own hub, and it does seem quiet restricted in terms of bandwidth, so unless you are struggling along with a .5 or 1meg connection, I’d go for it.