The goal is to map not just supply but demand. There's sod all data showing up on the site as I write (actually I suspect it's suffering a severe case of Beta right now), so have a look at ProjectX's blog post instead to see what they're aiming to do.
Meanwhile, their big competitor, Google, has rolled out an upgrade to Google Maps.
The former “hybrid” map view – which showed streets and features overlaying 2D terrain maps – with a new “terrain” view, which is shaded to show you hills.
It's certainly not something you'd go tramping with – it shows hill shading, not contour lines, so gentle slopes don't show up at all – but despite that and the low spatial resolution I find it does help give you a better sense of the terrain for everyday use.
(Google Earth still FTW, IMO.)