I actually have 5.9 Standard upgraded with everything even in the testing repository. Works fine, no problems at all.
I guess I shouldn't be doing it, but hey, it works. In the case of any breakage, I can always fix it or install VL again... it really doesn't take that long, and my data is safely backed up.
EDIT: Except for upgrading glib2. Didn't want to mess with that one.
I started slowly, only with those packages I wanted and only in extra...
Over time, there was not that much left to upgrade in testing, so I though what the heck.
Plus, I get to contribute to the testing
(I lie, there is one issue -- a bunch of errors when running vxconf and installing the nvidia driver --, that seems to be caused by upgrading the X packages. But those are actually in extra, not testing, isn't it?)