jetsam
Minister of Fire
I almost got one of those monsters because my old maul was bouncing off some tough rounds. I went to Northern Tool but they were out of stock so I held off and then discovered the X27. I'm glad I did because now, a couple years later, it hurts my shoulders to even look at a picture of the monster maul!
I haven't ever used one of those 15# ones, but I doubt you swing it like an 8# maul. Probably more of an assisted drop. (I kind of drop an 8# maul onto easy splits, and swing it like an axe for tougher ones.)
No matter what you use (including the x27), experience is required to do it effectively- knowing where the wood will split easily and where it won't is 3/4 of the battle. (Well, usually. People who split a lot of elm will probably disagree. . )