I have the 70-200 f4 IS and the 135mm L and thought about getting the extender but don't know how much I would benefit from it. I have read the 70-200 works very well but have not read much on the 135 L with it. Anyone have samples?

http://www.the-digital-picture.com/Reviews/ISO-12233-Sample-Crops.aspx?Lens=108&Camera=453&Sample=0&FLI=1&API=0&LensComp=245&CameraComp=453&SampleComp=0&FLIComp=0&APIComp=0

I bought a 1.4X specifically for the 70-200/4L IS. It slows the AF down and shooting at f/5.6 all the time was a nuisance. I haven't used it for anything serious in two years, even with my 200/2.8L.

It's worth it. It also works excellently on the 100-400 L, which I have, though because that is nominally f/5.6 as far as the electronics are concerned for AF, I have to manually focus it.

Personally, I'd use the 70-200 L at 200 without the tele-extender than the 135 with the tele-extender. Get the 1.4x II for the long end of the 70-200 so you'll have a very good 280 mm lens.