Yes- I've tried a demo of MojoWorld and I wasn't particulary wild about it's interface. It seems World Machine has a somewhat similiar interface- as does the Alpha of Terragen 2.

If E-on can deliver some really awesome looking clouds and atmospheres without the really geeky interfaces- that in itself would be an awesome accomplishment. However- I've never tried this MojoWorld plug-in myself- so I have no idea what the user interface is like...