This gives you the ability to climb! But you can't just scale walls like Spiderman: to climb a block, there must be a ledge for you to grab onto. 

To climb, just hold the climb key (Shift by default - rebindable in the options), and walk into a wall. If there's a ledge between the minimum (roughly knee height) and maximum heights (about half a block above your head) in front of you, you'll grab on. 

From standing, you can climb up to 2 blocks high, or 3 blocks with a jump. 

This is still very much a WIP - lots of things to iron out - but already working better than I ever dared hope thanks to the existence of vanilla climbing physics, meaning I didn't have to do half the work. :P