I'm a big fan of IS. The 70-200 f/2.8 is a heavy lens and towards the end of the day you'll notice an increase in your keeper percentage if you have it. I had the 70-200 f/2.8 without IS and may have actually kept it if it had IS.
It is a big deal. This was taken handheld just before sunrise with a 100-400 rather than a 70-200. There's no way I could have shot this at those settings without IS.
BTW, the new version will be released soon, so now is an excellent time to get the last of the current stock. You'll easily be able to recoup your money if you should decide to sell it later.
Canon 30D- Canon 100-400 - 1/30s f/5.6 at 400.0mm iso400