I have one and LOVE it, I see it as an additional knitting tool, and don't make entire sweaters on it (for fear that they will look like the ones in the instruction book).
For instance, my more recent sweater, I knit the front and back panels on the USM, dropped the center 3 stitches in front and hooked them back all the way the the top row (the Big Stitch technique in handknit style that basically puts one column 1 st = 1 inch in the middle of your 20 st = 4 inch piece) kept the stitches live at the top and bottom, handknit the collar with short rows, picked up and knit stitches for the sleeves on a circ needle and etc.
I hate matress stitch, and work to find other ways around it, so instead of traditional seaming, I picked up a row of stitches through each side and bound it off in the next row (kind of like using slip stitch crochet) Each side took 20 minutes to seam that way.
It is an awesome creative tool, and allows me to be more creative than I usually would in my handknitting, by eliminating weeks of boring stockinette
And it makes summer tank tops in about an hour.
If you get a USM, buy some Claw weights for more professional knitting machines. They are cheap (8 dollars a pair or so online, from the bond site as well) and you should use them instead of the silly rubber band and metal tension things they include in the box to keep the sides of your work from curling off of the machine.
I got mine on ebay for 70 bucks including shipping (there are always machines from people who used it once at xmas and hated it on ebay) and use a cacha cacha counter instead of the 30 dollar row counter they sell for it. I wouldn't have paid full price for it.