I tried updating a web running joomla 1.5.22 on Centos LAMP server, using the current version of JUpgrade (1.0.1) and it manages to download Joomla 1.6, but does nothing more.
I've found nothing in the components documentation which could explain this problem, but as we know, if one of us is having problems, there are more out there.
I know that the guys at Matware, along with some of joomla's most apt developers are working on creating the best migration tool out there, and for that effort I give them my thanks.
I will be following this tool and I will check out the next version. Hopefully it will be usable by that time ;)