The biggest use of Secret Passage is simply smoothing out your deck. Dump a village into the top of next turn. Keep the silver, send back the gold when you have $11 in hand. It is vastly more flexible than Courtyard so you can use it to kick back one of two terminals (e.g. 1 action with Smithy & Hag in hand) and it drops effortlessly into most engines. On the flip side it is exceedingly weak as a standalone play.
Late game it gets a second wind as you can eke out Duchies and Provs easier for a few critical turns by stuffing the bottom deck with green. Yeah when you get to those garbage hands they will suck, but very often you will have won or lost before you get to the bottom of your deck if you and the opponent both when engine, nabbed a few provinces, and are now tanking on duchies.
Unlike most sifting, Spass is actually pretty terrible at dealing with junk - it increases your search space by only 1 card for most things, but does not increase the frequency with which you see cards (shuffles are just as long and generally you still "play" each card once per shuffle). Smoothing is good, and it is nice not to lose draw like with a lot of sifters (e.g. Whouse, Cellar), but if you have garbage in hand it is often better to discard it rather than have to Gship it back somewhere in your deck.
On the other hand, it is great at setting up combo turns of low density/high impact cards (e.g. dumping back Kc x2 and Brg x3 from a 20 card deck into a single hand, or Nec, Smithy, Spass, Hgrounds, Silver, Gold, Copper, Hops). Doing that requires a high density of Spass's, but they are cheap enough to get in bulk from gainers.