As a maybe, slightly, just a wee bit biased, happy user of an x18, I can vouch for the upstroke power comment. When you are not used to it, the extra power is very noticeable, almost disconcerting. This is especially true with a wide stroke.
Connor
You know I was actually already thinking about the upstroke power increase from a dol-fin before you guys mentioned it.
The way I see it the dol-fin's ability to snap from one angle to the next without the delay flop of a bigger fin is a significant advantage for an upstroke user because you would get the full power of the kick from the very beggining, whereas a traditional fin is more powerful when the down stroke is a bigger push and the upstroke is a little softer and put's the legs back into primary position with less resistance.