This can never ever work for people intent on cheating themselves. Rated 5 out of 5 stars
In response to review 365169, I know how to get around this, however I also know how to drive fast and break hard, wasting petrol. Inexplicably I don't do things like that very often, even if they are fun. It must be because on some level I know it's not sensible to screw yourself over just because you can. Personally, I think that being intelligent and being intent on cheating yourself are mutually exclusive.
Anyway - review. This add-on is excellent. It prevents the casual "oh I simply must shop for a DVD box set of 'Allo 'Allo right now" moments. The add-on has various options - I started by using the "let me on for 10 minutes out of every hour" option but that didn't work well for me as I kept trying the browser to see if I could get back in yet. So instead I use the set times now: I have it set to block me for five ~1h blocks over a working day with 15-30min breaks in between. I actually find that when the block releases I often can't think of anything to waste time on for the break.
Note, it does not prevent you procrastinating completely, if you are of that mindset - for example, I am procrastinating writing this review. I should be working. However it's a great aid to productivity and I've recommended to to all of my friends who are similarly plagued with time leeches.