It's slick. I like the UI and especially the sets. However, sometimes I can't save sets. I set up the set, the extension tells me it is saved, I run the set and all the pages are opened correctly, but when I return the set is gone.

I can't see any pattern. Is there a maximum of the number of sets and/or the number of links in a set?

almost perfect!
please add Lazy Loading: loading the next tab after user specified time OR even better: keeping 2 (configurable) loaded at all times and loading another one when the 1st is closed

Fairly easy to add urls to a group, the settings popup disappears if you click outside of it making it tedious to copy/paste urls though. Ability to add multiple groups of urls is nice. One feature I'd like to see is keyboard shortcuts to open groups.