FYI - There's a good list of possible gotchas for the upgrade to MT 4.21 on the movabletype.org forums.
Additionally, it's been reported on the MT ProNet that Arvind's MT Blogroll plugin (whch I use to generate my hopelessly outdated blogroll at left) has to be completely wiped out or the upgrade will stall. Arvind has instructions on his site on how to upgrade MT Blogroll to his mew MT 4 plugin, Linkroller. I'm going to give that a shot and see what happens before I 'nuke' my MT Blogroll install.
Where you successful in upgrading to LinkRoller? I tried and it seems to have totally nuked my blogroll.
No, I did install it, but I couldn't figure out how to import my old MT-Blogroll data. I don't think it destroyed the data, but it didn't import either.
I haven't looked at it in a long time. Unfortunately, the wrap up work on my blog - like the blogroll - has fallen well down on the priority list.