I tried the 30/20/10 thing yesterday (as originally linked, I think, by Juicyju). I don't think I have ever done a harder run in my life. By the 5th 10 second sprint in each 5 minute set, you are really longing for the slow 2 minutes; by the 5th 10 second sprint in the 4th 5 minute set, you wonder if your lungs will ever recover. Interestingly, despite going ever so slowly during the 2 minute breaks between each 5 minute set, I recorded one of my fastest overall pace averages.
I recommend trying it.