I cannot begin to express how frustrating SFM can be to work with. It is a brilliant tool; easy to get into, but still loaded with options and possibilities, to help you create something unique, whether that be artwork or animation.
However (and I cannot stress this enough), I have experience on a multitude of occasions that my models wouldn't load correctly, or that SFM flat out refused to export my finished work. I have experienced that the lighting that I had strategically placed earlier, suddenly glitched out or perhaps perhaps just wasn't compatiable with the model I had in use. I must confess, however, that many a few of these "bugs" or "glitches" or just plain out errors that I experienced were most likely caused by myself. And while I find it frustrating that SFM refuses to warn me if I am about to do something that'll screw up the entire program, most of the time, I have come to the conclusion that really, it's all just trial and error.
Frustrating? Yes. Buggy? Yes. User friendly? Yes and no ... but a whole hell of a lot of fun to use? God yes.