I've been to all the other architectural salvage places in Seattle multiple times the past month and I've got to say that Second Use has the best prices. The selection is also pretty good, but they are not as selective about what they'll put on their shelves so you'll see a lot of stuff lacking in vintage or antique appeal. Meaning, there are 80s-style cabinets or cheap electric ranges. This is a good thing for some people, though, because the other architectural salvage places don't seem to carry anything but the really old stuff.
I've been especially impressed with their selection of brand new windows and tiles. The tiles are also displayed in a way that makes them easier to browse than at other places. They also have a bigger selection of sinks and scrap wood (including fun stuff like bleacher seats and wood from bowling alley lanes) than I noticed elsewhere. They didn't have a lot in the way of materials claimed from old schools or institutions, but I have seen a number of very large entryways in the warehouse. Now that I've got my cast iron clawfoot tub, I can also tell you that their prices on these tubs are about half what you'll find at Earthwise or RE-store. The doors aren't as well-marked as they are at RE-store so browsing through them requires more work on your part.
The store is pretty well-organized, although they seem to keep all the real antiques up at the front of the store, including items that you might expect to find in another department. As with shopping at any other second use place, you won't always find what you need, but you should check back often. Also bring with you a healthy dose of patience. Final note: a larger percentage of their inventory is outdoors than at Earthwise or RE-store, so that's something to consider if you're stopping by on an especially rainy, sunny, or cold day.