It is far better than browsers native bookmark features. However there is quite a bit of resource usage during normal browsing (not when sending tabs to onetab or loading onetab tabs) according to firefox's task manager. It clearly isn't just jotting down URL's when you press the button which is both a privacy and performance concern.
Also if you restore tabs from onetab too fast (and have it set up so tabs are deleted once restored) you lose tabs.