I keep two blogs on WP. Just as an experiment, I tried to upgrade from version 2.5.1 to 2.6. Downloaded WP 2.6 from the official site.
Impressions: good : all plugins work, the blog itself seems to be working a little faster (did not measure); good, but hardly applicable in my case : you can return to any version of the post when editing it repeatedly; a clear plus : a red pop-up “antung”, reminiscent of drawing attention to new comments, is now above the “Plugins” link; strained, but you can live : when inserting a picture into a post, not just the img tag (with a link, class) is inserted, as it was before, but a much more voluminous and otherwise displayed in the public part html-code; some kind of nonsense : all downloaded media files tend to the wp-content / uploads folder, regardless of the settings in the admin area (it was - to the files folder); very bad : it is impossible to upload a picture when writing a post — neither when using the “flash loader” nor the “browser loader” (options of the media download script).
Perhaps, the hoster also played a role here (Peterhost - they need to prescribe something in .htaccess, that the scripts normally loaded files via http - checked, everything is normal with .htaccess). As a result - a negative impression, temporary costs for rolling back to the previous version. ')
ps.: While writing the post, rolled back the WP to the old version, tried to enter the admin panel in the tab - FF issued crash, the text of the post was lost, I had to re-type :(