Easy controls, much customizability, easy to get into, pretty stable (only crashed on me like 10 times total in my current use time) and produces amazing results.
Sometimes you have to deal with some really annoying problems, like fixing broken materials or what not, but that's not too bad (and only is the case for TF2 stuff, really).
Only thing that'd maybe help newer people are some fixes to item painting so that you don't always have to 1. use a script and 2. not dig around the material files. Same can go for unusual effects and adding special variants of weapons (festive, botkillers, multi-model and multi-style items, ...). But that's all just focused on TF2 stuff.
If you are an eager movie maker or artist that wants to use assets from already existing games, this is the program for you. The community is also very helpful about any questions you have!