In fact i have same question as well, because i don't have enough money to buy HC 120 macro, and my HC 80 is not great for macro shots, so i was thinking about extension tubes, so before yesterday [Tuesday] i went to a local dealer of Hasselblad/Profoto as he told me that they had bringing some extension tubes, so i went there to test them [they have only ET 13 & 26] and he told me that the company [i think he meant Hasselblad] told him that 52 is not favorite or not recommended, anyway i tested only 26 that time even i saw 13 but really i don't need to test 13 and that 26 test was enough for me to buy it immediately, so now i can do some macro shots by using extension tube 26 with 80, cheaper alternative than buying a Macro lens [but the lens in my list anyway].
Good luck!