When I was looking for a new templating system for Roundup, I settled on ZPT pretty quickly. I have the occasional Roundup user who complains about the complexity (while, at the same time, being able to get the job done), but most users are happy with it. The i18n stuff in ZPT can be fiddly though.
Personally, the ZPT design (TAL, TALES, METAL) just work for me.
One issue that's come up in Roundup though is templating of plain text. As Jeffrey points out, non (HT+X)ML is something that ZPT is definitely *not* targetted at (and is why DTML persists in Zope, as you point out). Still looking for something elegant there -- I wonder if anyone's implemented a perl-format-alike in Python...
Note that SimpleTAL isn't a complete implementation, and isn't as mature. It's definitely usable, and is getting more mature though.