Well, actually I was shopping for a 200/2.8L.
I'd recently bought 135/2 and already had the 1.4X II... So I thought "what the heck" and gave the combo a try.
I immediately decided not to buy a 200mm. At least not for now. Maybe once I save up enough for a 200/2 IS.
Not that the 200/2.8 is a bad lens. Lots of folks seem to love it. I just don't see a need for it in my kit, thanks to the 135 + 1.4X combo.
I have heard that the Kenko "300" 1.4X rivals the Canon 1.4X II for image quality with some lenses. But I'm not sure of this particular combo or made any direct comparison myself.