This is great! Not only does it toggle collapse state of comments, it also helps me quickly pinpoint how many layers deep I am within a thread. Unlike similar extensions, this one produces a thicker highlighted bar - which improves clarity substantially. Nice job, dev!
update (01feb2019): I should point out that I'm also using "Old Reddit Redirect" (extension that redirects me to the older design: old.reddit.com). Worth mentioning because this extension's last update was Jul 21, 2017. Working great on the classic design! Untested on whatever trendy new design reddit is fudging around with.
Great add-on. Makes easy to collapse comments by clicking on the sides... and also helps with navigating comments in conversations.... really useful tools. So much useful that the new reddit design does what this mod does... but who wants to use the new reddit. Am I right.
This makes it much easier to get out of a nested rathole of off topic comments and back to the interesting stuff. A simple UI that does the job well. This should be a feature of Reddit, or at least RES, but until then, I highly recommend this add-on.