The RSPB shop had these on their windows and they were great so I bought 2 and SIL bought one. They never attracted any birds in this garden or my last. Fell off quite often too. May work in some situations but I had no luck with them.
If the feeder is on a window you don't go to very often, it should be fine. We get bluetits and a wren up near the back door (when the catz aren't about...)
on a window with a net curtain they work better but that defeats the object rather... blue tits and robins are pretty brave they will get there eventually
Yes I think they'd get used to it..I don't have a window seed feeder, I hang them in a tree, but I put raisins on the sill and the blackbirds never stop fetching them!