Yesterday evening (BST) Peter Stephenson released version 5.0.0 of the Z-Shell (announcement with download information). It is the first stable release in four and a half years (version 4.2.7) and the first major stable release in more than eight years (4.2.0).
A comprehensive list of changes can be found in the `NEWS' file within the distribution. Some of the highlights are vastly improved POSIX emulation and of course the support for multibyte encodings (like UTF-8) within the line editor.
I'd suggest you try the new release and let the development group (the -workers mailinglist) know in case something goes wrong.