it looks more like a world editing suite and game engine sort of thing, rather than a modelling prog. reading through some of the blurb says that it supports importing some model types, probably in much the same way that hammer/source does. it also mentions some other stuff that i know nothing about. visual studio, c++, etc. all the same sort of things that ReNo said he'd been using to write his own game for a uni project, so maybe he could give you a clearer answer on what it really is.

unless you've got a wooden leg and a parrot named polly, or have more money than sense, there's only two real choices that i know of to model for hl2. milkshape and XSI. milkshape because its cheap as chips, and XSI because it's even cheaper. there's blender too, but it's limited in its export formats, and you'd need milkshape to do the conversion to .smd format. (milkshape=king of the converters)

in milshapes favour, i'd say it's got a lot less bells and whistles, so the learning curve, although steep to begin with, is still much less than the more complicated XSI. and in XSI's favour, it's a far, far superior modeling tool. for animating player models and use of vertex weighting, it laughs in the face of milkshape.