I tried out the Seer on all his 3 functions: advice, activating the shrines, and replacing lost items.
(1) Advice I found his advice to be accurate but very general e.g. something like "You need to go and visit the Castles" (I can't remember the exact words). Not really very helpful.
(2) Shrines Activating the shrines I have found to be spot on – totally reliable.
(3) Lost Items I found the function of replacing lost quest items to be unreliable. It worked occasionally, but more often than not it said "You never had it" or gave me a second item that I already possessed. I've not used it for this function since my very early days with the game.