I've got my Sky+ box connected using BT Internet.
Is it good - yes.
Is it fast - yes it can be.
Is it always fast - no.
Who's at fault - Sky and peering networks IMO.
The issue is Sky's service infrequently suffering service issues (through server or network links). I can normally watch HD content within a couple of mins but I have found several times (even outside peak hours) that it can be awful.
I suspect it's probably a combo of Sky's service and network peering links having issues as having tested my broadband line out when having issues it was fine.