I'm pushing 50 and until a few years ago, I split everything by hand. For the longest time I used an 8 lb and 6 lb maul, and a 4 lb ax (can't recall the brand). I finally broke down and bought an X27 and was absolutely shocked at how effective it was. There was literally no more need for the 4 lb ax or the 6 lb maul. If it didn't crack with 3 or 4 swings with the X27, I went straight to the 8 lb maul. I'm cutting mostly hickory and oak with some occasional locust and ash mixed in, and most of what I cut starts out around 20-22" in both length and diameter, but I get some occasional stuff that's closer to 3' across. I'm 6'2", and I never bothered with the X25. I split on the ground and not on a chopping block, so the X25 just wasn't going to work for me. I could never understand the benefit of having to pick everything up an extra time to set it on the chopping block. With that said, kindling is a different story. The X25 would be great for kindling IMHO.
A note on breaking wood handles. It's not always an over strike that causes problems. Sometimes the wood splits in such a way that it curve back under the handle causing similar damage. With a maul, if you're using a wood handle, make sure the maul uses the axe eye handles instead of the round eye like a sledge hammer does. My 8 lb maul had a round eye, and I couldn't get more than 2 seasons out of it (4 cords/season) without snapping the head off, and that was WITHOUT any over strikes or other damage from making contact with the wood. Even with the proper grain, a round eye just can't take the abuse from an 8 lb maul on the end.
The same year Fiskars released their 8 lb splitting maul, I started having issues with my rotator cuff. After about 2 hours of splitting one night, I couldn't swing a maul or sledge for the next 6 months. I picked up a used 27 ton splitter that summer and never looked back. I still split a lot (ie most of my wood) with my X27, but its very rare for me to swing a maul now. If it needs halved or quartered, and isn't very knarly, I split it with the X27. If it hasn't split after 3 or 4 accurate, well placed hits, it gets rolled to the side and split with the hydraulic machine at the end. If its trying to split, but the X27 just isn't quite enough to get the job done, and I only have 4 or 5 pieces like that, I MAY pick up an 8 lb maul to finish them with rather than mess with starting the splitter. If the round is too heavy for me to man handle onto a splitter to begin with, I use a 90cc saw with a 28" bar to noodle it into more manageable size pieces.
With my shoulder issues, I never bothered getting the ISO tool. I look forward to hearing about your impression of it compared to the other tools.