I have one! It makes a shade under $40 per month.
As for what people find valuable, though... I honestly don't know. The two folks on the Super Tier are there because they love my comic - they could get exactly the same perks through the Read Ahead tier, and they've never taken my up on any of the things that the Super Tier mentions they can ask me for.
And the three folks on the $1 tier are silent 'lil munchkins, who I'm pretty sure just think of it as a tip jar for a comic they like. (Which is exactly what that tier is, I guess.) I pop up sketches and the odd completed panel from future episodes for them.
I get the impression that none of my Patrons particularly care what I actually put up on the page, they just want to help support me in making my comic. 