I picked up the 70-200 f4 for about $450 from Canon Refurb when they had 20% off. With that said, I live in sunny Florida and I use the lens exclusively outside, so fast shutter speeds are very common. I love this lens and seem to get a good DoF with f4. Could it be better if I had the 2.8? Sure, but my wallet just didn't want to cooperate with me.
And not to throw a wrench into the mix, but you might want to consider the 135 f2 or 200 f2.8 primes. They are both cheaper than either the f4 is or f2.8. If you just need the reach without the need for zoom, then just get the prime and you'll have better, faster glass for less money.