Short bio: Computer Scientist, FOSS supporter (read more)
Tux Machines (TM)-specific
First lets start with the good news. The gui is much more responsive. Interacting with the sidebar is a smoother experience. Not only has the underlying code obviously been reworked but the look and feel has been overhauled too. It fits in with my GNOME desktop better than before.
Now let's take a look at some of the bad, or more accurately, the ugly. The previous version didn't use many native GTK widgets. The only native GTK looking part of the previous version was the media controls. Now it is one of the few parts that does not respect the native GTK theme.