Lance writes ...

Has anyone used a Canon 70-200L F2.8 lens with a X2 converter?

Yes, but only when I don't have access to my wife's 400 f/5.6 L or my
300 f/4 L with a 1.4x ... in other words, in an emergency. I try to
stop down to f/8 or f/11 when doing this.

what is the image quality like? Still pretty good?

Adequate but not very good compared to the 400 f/5.6 L. Here's a link
to a site that tested the 70-200 f/2.8 L IS with the 2x vs the 100-400
L IS and the 70-200 w/ 2x lost. The 100-400 IS is the worst lens in my
kit so you get the idea ...

http://www.luminous-landscape.com/re.../400v400.shtml

Bill