Upgrade went smoothly, except for a small login error and a couple minor plug-in errors.  I Google’ d it like usual, and came across this blog site that had the EXACT same errors I had:

http://konstruktors.com/blog/wordpress/169-solving-authentication-and-plugin-errors-
after-upgrade-to-wordpress-2-5/

“You do not have sufficient permissions to access this page” after Admin login, and also the fatal error on activating the Popularity plugin.

To fix the “You do not have sufficient permissions…” error, I had to re-create the WP-Config.Php using the WP-Config-Sample.php file included with the new version of WordPress 2.5.

Popularity: 100% [?]