Your subscription entitles you to one ticket for each production. You get to book your ticket before tickets go on sale to the general public, and you can exchange your tickets up to 2 days before a performance. I'm not sure what you mean by perks, but Signature offers a discount at their bar and their bookstore to subscribers.
I've been a Signature subscriber for 10+ years (since before they moved to their new complex) and I think it's definitely worth it, especially if you plan to see everything they produce in a given season.
"You travel alone because other people are only there to remind you how much that hook hurts that we all bit down on. Wait for that one day we can bite free and get back out there in space where we belong, sail back over water, over skies, into space, the hook finally out of our mouths and we wander back out there in space spawning to other planets never to return hurrah to earth and we'll look back and can't even see these lives here anymore. Only the taste of blood to remind us we ever existed. The earth is small. We're gone. We're dead. We're safe."
-John Guare, Landscape of the Body
