I use WhenX since the beginning. It's very usefull and I really take advantage of it. It's one of the things that you wonder we nobody thought before and why isn't this feature included on every browser yet.
I spend 2-4 hours a day doing research for my job. I see so many websites my eyes glaze over and it's impossible to remember where you have traveled on the internet. WhenX is like the "mile markers" on the US Interstate Highway System. It gives you visibility into where you have been and whether that website was useful or not (by using the notes.)