Being able to consume media without the fluff of sponsors, being reminded to sub, intros and outros built into the video makes this a special extension, especially when you can do it on normal videos and music videos. Not only is this a crowdsourced extension that everyone contributes to, but you can too! If you see a video not have the sponsorblock added, please contribute if it needs it!
It's fantastic at what it does and I cannot recommend it any further than the highest praise. Make sure to check out this developer's work. Very smart, very helpful and very user-friendly!

10 months later:
It seems I cannot mark a full video as "Unpaid/Self Promotion" without it going away upon refresh. This extension was phenomenal before, it can be again.