I don't mind them from a privacy standpoint (they're less detailed and less identifiable than your average X-ray!), BUT:
My home airport has 5 regular metal detector/puffer combos (that each serve 2 lines) and 1 full body scanner (that has 1 line). It is totally optional which one you go through. When I went to WDW last year, there were only 3 people in the full body scanner line so I figured I'd do that. So I was the 4th person in a line going through 1 machine, my DH was the 8th person in a regular line.
It took me 25 minutes from the time I got in that line to the time I got through. DH had been waiting for me for 15 minutes and was afraid I'd gotten hauled off by the TSA.
That means it takes an average of 6 minutes per person to go through a full body scan and a bit over 1 minute per person to go through the metal detector/puffer.
Which means that going through security once every airport is using full body scans may take SIX TIMES AS LONG AS IT DOES NOW. Which means that at, for example, O'Hare or Newark or LAX, you may be standing in that security line for HOURS.