Common insults in the UK Armed forces:

"As much use as a rubber crutch"
"As much use as a concrete cushion"

One of mine:

"Useful as a haemorrhoid transplant"


I believe that Georgiansilver said, "... a computer without a keyboard". Quite apt. Try booting a PC without a keyboard, and it will often say, "Keyboard error. No keyboard detected. Press F1 [or any key] to resume"