I upgraded from Wordpress 1.5.2 to Wordpress 2.0.2 this morning.
The servers that power unto.net have a relatively small amount of memory available. I had found that even a single request to WP would instantly balloon the resident memory up to 25MB from the 7MB baseline.
Since Wordpress 2.0 has caching built in I hoped that would address the concern. So far, so good. Per process memory is down, at least for static WP views. The first view of a static page now pushes the process' memory up to 13MB, 7MB of which are shared. Editing a post (i.e., hitting wp-admin) seems to push that to somewhere around 20MB. (By the way, this is absurd, so I'm not sure what's really going on.)
The AJAX-style WYSIWYG post editor is acting a little wonky for me -- paragraph breaks in particular are hard to deal with -- and I haven't bothered upgrading all of my plugins. Otherwise there doesn't seem to be a downside to the upgrade. Akismet still works, which is key.
For what it is worth, I didn't upgrade "in-place" the way the Wordpress documentation suggested. I set up a parallel directory for the new installation and upgraded there. This instantly broke my old version but then I simply flipped the symlinks around and the users were up and running. The Wordpress developers did a great job making the upgrade process painless, even for someone who had made some tweaks here and there along the way.
On a related note, I did an install of MediaWiki for Unto.net today. I haven't opened that up to the public yet, but the plan will likely be to replace the Unto.net Wordpress blog with MediaWiki. I would like to allow registered users to feel free to make edits if I leave something out of a post.
Please let me know if there are any issues with the new site.