The 2X was designed to work best with the super tele primes and it's amazing on them. It's also very good on the 70-200's but as Joe mentioned it automatically loses 2 stops so your 70-200 becomes an f5.6 lens. This is not the end of the world if you're using it outdoors and you have IS. The 2X on my 70-200 f4 L IS well even though it's f8 still AF's on my 1 series.
The first jet ski is a shot with the Canon 2X on my 70-200, the second is the 400 f5.6L with Canon 1.4X and the bird is done with the 2X on my 400DO which is what I bought the 2X to use with.
(Message edited by richd on January 17, 2009)