I have a wide foot size 9 with a very high instep and flat foot.
I did order these one size up on the advice of many reviews on here.  Putting them on for the first time, it was a very snug fit due to my high instep and the fact that the strap is rigid without any give to it.  Width wise they would have been fine but if they were to fit my instep, the length by ordering one size up would have been too long.  If you have a narrower foot with regular or low instep then I would say to order your true size.  Being that I have a flat foot, the instep was very uncomfortable and the foot bed feels very rigid (I prefer a bit more 'cush' in my flip flops).  I have now worn them several times a week and while I feel they have stretched or adjusted a bit to my foot, they are still by no means anything I would consider comfortable.  For comparison, so far the most comfortable flip flop I have worn is an Adidas Raggmo.