Seemed nice, but when I've first time changed to a group of a few selected from many tabs it hid them without an easy way to change back to the ungrouped ones.
I've found them under the "v" tab menu and saw an option to change them into a group, which I used, as I saw no other way to get back to tabs without groups. After a few minutes of CPU abuse the "Ungrouped" group had zero tabs. 400 tabs went straight to /dev/null.
So yes, a good way to clean out your stale tabs, short of removing your profile.

Edit: Now I'm torn, as I've found out how the "Manage groups" looks and works.