As the title say: has anyone any experience growing these? I have one that did great this year but seems not to be doing so great now and there's really no information about Podophyllum hexandrum out there.

Did great for how long?
Years?
Or just a few months since planting?
As you aren't giving us information about growing location, I'm going to have to guess at the apparent problem...
The mayapple varieties that I have experience with go dormant as autumn comes on...

I'd expect this one to do the same.

I bought in late April and it started sprouting new leaves like crazy immediately after planting. It got a little hail damage in July but it didn't affect the overall health of the plant. About one month ago I started getting big grey spots on a few leaf margins, about a couple of weeks ago the new growth just wiled and died.

It's been a very dry fall: it hasn't rained in several weeks so I keep on watering, albeit less often than before. Night temperatures are in the low 50s now, the plant is planted behind the house so it doesn't get the full heat of the sun but it's probably low 70s there in the hottest part of the day. I removed the straw mulch last week.

That's about it.

I really like this plant and I am thinking of buying another next year (big money) if I don't kill this one as well.

I wouldn't have removed the mulch!

I'd certainly put that back... Otherwise... I'd let it go dormant.

You still haven't given us a location...

I'd actually suggest growing some of the more common types... Knowing what to expect from experience with the native types is always helpful.

These are commonly available:
https://plants.ces.ncsu.edu/pl...

These are locally native all over the eastern part of the country:
https://hort.extension.wisc.ed...

As both of these colonize like mad where moist enough... I wouldn't expect a need to buy additional plants of the one that you have....

Sorry for this late reply. Podophyllum is the old name. Himalayan mayapple has been reclassified as Sinopodohyllum hexandrum. Perhaps you can find more info then.

The grey edges are just a result from some time(s) of excess dryness. Even if it were a disease, I would worry about it, but I would dispose of the affected leaves in the garbage to be taken away if you think it is.

As has been said, your plant is naturally going dormant for the winter. In a hot or otherwise less desirable location it would be normal for it to go dormant much earlier, even in mid summer. So I think you are doing well.