I love that tileset and use the hell out of it. But the vertical distance between each level is quite small, and there are walkmesh issues requiring the use of placeables to channel players so they don't get hung up on the mesh, because it's not always apparent what's walkable and what's not. So right now, if you want to do a tunnel really going DOWN, you have to get wild and crazy with those new walkmesh helpers from last year and rocks on blank tiles in order to create a real sense of three-dimensionality. (and yes, you can do that just by making an "exterior interior," which is great for a module guy, but in pw-land where I live, that's problematic because of the huge size differences between exteriors and interiors -- your online module can have scads of the latter for every one of the former).
I'm probably in a distinct minority on this one, though, so if it's not of interest I'll just stick with my walkmesh-helpers for building the truly crazy-deep stuff. =)