I just did an almost-complete incremental upgrade / reinstall of my
production system (dpkg --auto --no-also-select --refuse-downgrade,
but not --skip-same-version).
The whole operation went fairly smoothly, and produced only the few
mostly minor bug reports that I've already sent.
The most serious problem was the xbase bug, but even this can be
considered a blessing as it uncovered a (not very serious) bug in dpkg
For the 161 packages I have installed (most of the main packages, but
no TeX) the operation took 40 minutes, including brief amounts of time
spent at about a dozen postinst prompts.