I've been using it since it came out and have had only one problem. My Multi-row Bookmarks add-on wouldn't work. A new version of it compatible with version 4 was released, but it's somewhat buggy. Other than that, no problems. I have noticed that V4 is way faster than 3.6 and previous versions.
Maybe the tab grouping (fancily called "Panorama") would work as a replacement for multi-row bookmarking?
By the way, I really like that feature. It's the single most important improvement from 3.x to 4.x in my opinion. I've had an unmanageable amount of tabs open at best (over 200) and the groups help very significantly in keeping that amount in control.