I'm a big fan of my X27. It handles nearly everything I throw at it. I've found that it's counter-intuitive to over-swing it, though. At first I was starting with a full 180* swing (from behind me all the way in front to round of wood) but somehow it wasn't as effective or accurate as a 90* swing (starting holding it straight up in the air and coming down on round) More speed, less power, and keeping the blade sharp really makes all the difference with these guys. Slightly modified technique from traditional, heavy maul.
The only thing I have trouble with is really knotty, twisted stuff and rounds that are over 22-24" in length. I've split a bit 24" length oak rounds with it this past summer, but it certainly wasn't easy. Under 20" and not knotty is a piece of cake normally and it doesn't wear you out like a maul does. I split about 3-4 cords this spring and summer with mine.
I've definitely gouged mine up a bit from hitting the ground. I was really careful at first to try and split on top of another round, but found it better to split with the round either on the ground or really close to the ground for the extra swinging distance. Somehow, the little plastic handle sharpener (make by Fiskars) actually works to restore the edge on the blade. Doesn't take out big chunks or nicks, but I suppose that's what a file and a lot of patience are for...
Also have the little X7 hatchet, which is still quite sharp and the blade is still in perfect shape. Great for splitting kindling. I'd love to have another X27, just to start out fresh with a new, untainted blade...