Today I upgraded from 2.0.1 to 2.7.1 and that went pretty well, no major problems that I can find.
The whole point of doing this was to acquire the export function in WP in order to simplify the export of the blog in anticipation of a migration.
The problem is that running export produces an incomplete XML file. For one thing, it's 17MB where the sql backup of the db is 200+ MB.
It contains as far as I can see, no posts and it seems to be made mainly of spam comments. (Askimet has been activated at this blog forever).