Wow, I've been writing e-mails all morning! Not that any of them are useful.
I would consider gui at 1.0 when the cairo backend gets completed. It's already very good in my opinion (kudos to Fred). And have full support for at least PNG (wasn't there a GSoC for this?), as well as make PNG the default image file (this includes converting the current images).
The little that I've worked with gui when going through the different tutorials and writing my little ScreenSaver framework (which went on hold) was great. Of course, I didn't use gui very deeply in either case.