I've only used a knitting machine for knitting thin metal wire while working as a grad assistant to my metals prof. She wrote a book called "Textile Techniques in Metal" and I was using the machine to knit her samples for her.
I think they are faster if you know how to use them. I could definitely crank out a nice knitted piece in a short time. I'd be wary of the cheaper ones, just because of stability issues. If something gets stuck, cheap plastic hooks will break or the legs will wobble.
I got a knitting machine this year. I thought I'd really use it for large knitting projects, and then do lots of felting. I had grand visions of making wool blankets, etc.
But the reality is, I love the versatility/portability and the look of hand knitting. I also find it much more relaxing and fun than working with the dang machine that ALWAYS seems to frustrate me somehow!
I'm sure someone here must have good things to say about them, though!!