I upgraded today to new Wordpress 2.5 from my old version 2.3.3. The update went painless although I use a lot of plugins and took just around half an hour to complete. I recommend using the Automatic Upgrade plugin which should save you a lot of trouble. I have also updated my plugins, SEO Friendly Images, Theme Test Drive and Smart YouTube to new WP 2.5 design. Frankly I do not fancy it a lot, somehow I got used to the old one, specially the dashboard but time will tell.