I first read about tags (where else) on Giftie's Blog. She uses them to mark her project pages and I held out buying some for awhile because I wasn't sure I would like how they stick out, and I was also worried that I would pull them off every time I opened a particular page. I use A-Z dividers on my "Files" section but using a second set would make my planner to bulky. I finally gave in and decided to try them. As you can see below, they don't stick out too far at all. They've become welcome additions to my planner.
Check out this post on tags from Chaos to Order.