I've used knitting machines. They're really good for doing plain knitting (stockingette) or simple colour patterns (like stripes) in stockingette but for more complicated patterns, lace patterns and cables they're not much good as you have to manually move the cable, form the loop for lace etc. and it's too time consuming. They're ok for rib once you've gotten the hang of it all, basically each stitch is held on a hook and the position of the hook determines whether it is knitted or not. If you just wanted to make a jumper though, you could do one in a day no problem once you'd gotten the hang of it.

Bx