We took the fully furnished option - mainly because we had so much other crap to deal with outside of EK that we just didn't want the hassle. Most of the furniture is fine but I do agree with the coments about mattresses. They are like slabs of concrete and do need replacing if you want a good night's sleep.
If you have the luxury of spare time (a rare commodity in EK at the best of times) then I would recommend taking the allowance.