I haven't presonally but have asked my dealer as he keeps both 330i and 770i on hand for people to try and has heard the 550. He mentioned the reason he has the 330 and 770 and no 550 was because there wasn't enough of a disernable difference between the 330 and 550 or the 550 and 770 to justify the expense of having it on hand. I still haven't done the listening test but he says there is a noticeable difference between the 330 and 770. Basically the 550 blurs the lines between them which makes it harder to sell over the 330. I suppose it also would make the 770 harder to sell over the 550 He definitely has customers that order the 500 to save some money over the 770 because they want that bit extra performance over the 330 but don't want to pay 770 prices. If his cables were balanced I would bring them home to test in my system but since I'm setup for balanced I would have to recalibrate to incorporate them into my surround setup as he only as a pair of each.
I suppose for longer lengths it may make sense to get say the 550 over the 330 simply due to the heavier gauge wires and that could be true of the 770 over the 550. This is something I'm going to have to think long about as I'll eventually be needing somewhere in the range of 40 feet to get to my surround speakers when I get mono blocks for them.