I have been hearing horror stories across the board with WordPress 2.0 release. The wordpress forum is filled with wide ranging issues. Here's what you can do.

If you are on WP 1.5.x version then I strongly suggest that you wait for a patch release with all the major issues sorted out.

Robert Deaton came up with a list of issues with WordPress 2.0:

http://trac.wordpress.org/ticket/2168
http://trac.wordpress.org/ticket/2154
http://trac.wordpress.org/ticket/2167
http://trac.wordpress.org/ticket/1917
http://trac.wordpress.org/ticket/2164
http://trac.wordpress.org/ticket/2163
http://trac.wordpress.org/ticket/2162
http://trac.wordpress.org/ticket/2160
http://trac.wordpress.org/ticket/2161
http://trac.wordpress.org/ticket/2130
http://trac.wordpress.org/ticket/2159 - This issue was reported on testers four days ago
http://trac.wordpress.org/ticket/2156
http://trac.wordpress.org/ticket/1447 - This one really should have been fixed, skippy posted to the hackers list, skeltoac uploaded a patch to it over a week ago, there’s no reason for this not to have been committed.
http://trac.wordpress.org/ticket/2153 - Why in the bloody hell was this pushed back?
http://trac.wordpress.org/ticket/2155 - A direct side-effect of too little time in testing
http://trac.wordpress.org/ticket/1577 - I still don’t see why this, or WP-Cron haven’t been built in. There have been patches for these things for months.
http://trac.wordpress.org/ticket/1388 - Once again, months have gone by.

If you have already taken the plunge and upgraded(?) to 2.0 and facing problems then you can use the downgrader script (by Robert Deaton) to convert your WP 2.0 install back to 1.5.

Despite all the warnings above if you still want to upgrade then please please please (repetition intentional) backup the database and harden your templates against plugins before you upgrade.

It was a release done way too soon. I am personally waiting for a patch before I even think of upgrade.