Interesting firefox add on

After my blog post about pinterest and a recent post by Marcia François, I'm starting to see a trend here: internet can be a HUGE timewaster if we let it be. I was going through some old bookmarks and I stumbled upon this one: leechblock. First, I love the name! Second (and more seriously) the goal of this add-on sounds interesting. From the add-on page:

LeechBlock is a simple productivity tool designed to block those time-wasting sites that can suck the life out of your working day. All you need to do is specify which sites to block and when to block them.

What I find interesting here is that it allows you to block certain pages. I'm thinking Facebook, flickr, pinterest, all those fascinating websites where you can easily spend your day mindlessly browsing. It also allows you to block some pages during certain periods of the day: if you want an extra help to avoid timesucking sites during the day yet with freedom to go there during the evening, it sounds great! I think I'll install it and let you know how it goes!

What do you think? Would such an add-on be useful for you?