Take it as you will. I'm not a pro mapper like some folks, but a of friends told me about this other day, and I decided to toy around with it a little then:
I'm fairly proficient at bsp, and even so, some of the more awkward parts of bsp are pretty much fixed by this editor. For example, the ability to just just drag to create some odd shaped terrain instead of using all clipping plains made the process insanely fast. The 2d windows in bsp are nice for laying the map out and getting a feel for where you want things. But a finished map seems like it could be plugged out of trenchboom in a fraction of the time, and with better aesthetics.