I've posted my experiences with Elm a couple of times. There aren't many large American Elm trees here on the east coast but I was asked to take a large, healthy one down a few years ago before the old mill building it was next to became condos. When split, the wood looked like the pictures posted by Z50guru.
My splitter worked though it but every piece had to be pushed through completely. I actually used a block of that tree as a pushing block on my splitter. It was strong but at the end of the next burning season I decided I just didn't like Elm as a firewood. Didn't start to burn well, took a fair amount of heat to keep burning, not even worth stacking in the pile. Well, didn't that get a few replies. Seemed folks in the middle of the country burning pieces from large, standing dead Elm trees love it. Hmmm... That's worth thinking about. I left some to season out for another year and tried again. That wood burned better, still nothing to rave about.
Since that time I've taken down some smaller standing dead and what a different experience that is. I just threw a piece on the fire that seemed nearly the same density as a piece of Oak beside it. It burns hot and lasts for a decent amount of time. If you get the wood when it's decomposed just a bit, there's no stringiness at all. It will dry quickly after stacking and probably will be ready to burn in the same year its cut. So I can see why some folks love it. I probably won't be asked to cut an American Elm as large as the one next to the mill building again, but when I do get green Elm now I just set it in the "long time seasonin" pile. Couple years at least before it goes into the woodshed. That wood loves to pull moisture out of the air and it needs to be dry to burn well. On the plus side, I'm still using the same pusher block five years later.
Oh, and if you happen to get Siberian Elm, don't bother. It makes Cottonwood / Poplar seems like a high value species. It doesn't want to dry and it rots fast. When dry the only thing it does well is take up valuable space in the wood pile. Siberian elm has very rough bark and a reddish-brown heartwood that looks very beautiful.