I have just started using impact today and it seems great so far, but it seems odd that support for slopes is missing from the Box2D plugin. Fortunately I found a thread with a fix for this http://impactjs.com/forums/code/box2d-collision-polygons-for-new-slope-tiles, but it would be nice if there could be official support for slopes and possibly other oddly shaped tiles in the Box2D plugin.