So, I just did the upgrade.
Everything got smooth except for the updating of one language pack. My forum has a Dutch language pack alongside the regular English one. The upgrader script succesfully detected 2 languages and upgraded the English language but then hung. I ended up just pressing the back button and choosing for the manual option. After upgrading, I couldn't easely get into the control panel as it gave me a white screen. I figured this was due to the broken Dutch language pack being the default and it not recognizing the new Dashboard page. After manually going into the language cp panel page and disabling the Dutch language pack, it all worked fine. Only the language editor will give me a blank page as results when searching through 'All Files' as it will likely try to search the disabled Dutch pack too.
All other new features seem to work fine. The only thing were the layout of the showflat/post next/previous thread buttons, which didn't go well with my custom CSS. These apparently had some style (padding) set through the template so I had to change that. Maybe it's an idea to do all styling through classes in the user CSS instead of in the template files? Even if it are silly things like padding.
Besides that, I'm really happy with all the new features and cleanups. Great job on all the hard work finishing this version the past couple of months. Really appreciate it!