I seem to have inherited my son’s BT Vision box (and him too), is there anything on there worth watching? (I already have Sky).

No

No, it’s pish.
The box doesn’t work in my experience.
YouView is much better.

Does the BT vision bit work even without you having an account with them?
If so, there are some “classic” sports things which are free, e.g. MotoGP, boxing etc. I’d be surprised though if they work without a BT account though.

Does the BT vision bit work even without you having an account with them?

No

Viewview is the box (that you get now) which is an OK Freeview HD Plus box. BT Vision is a service installed on the box which can stream BT content to your TV. Content is a bit of a strong worn and a lot of what there is has to be paid for.

If you have BT broadband and want a Freview plus box then Youview for £5 a month isn’t a bad deal but that is all it is.