Been using mine for a while now!
The bag zips off the base, which has a hole in it, making refueling a doddle
They're fixed to the bike fore and aft, and can move about a bit when fully loaded, so make sure you fit it as tightly as possible.
It hasn't marked the tank (yet)!
I took some piccies to show where I fit it under the seat, but am waiting for my kids to get home to dowmload them (technophobe me ).