A concensus on teleconverters.

We talk about lenses at great length, but it seems rare that we discuss the optical quality of TC's. So whats the best 1.4, the best 2.0? Is it Canon, Sigma, Kenko, Tamron, somebody else? Who offers the best bang for the buck? What are you using?

IMHO it depends on the lenses they are used with.
For the Canon big whites, the Canon TCs are best IMHO.
I used SIGMA TCs exclusively with the better SIGMA lenses,.

I have both the Canon 1.4 and the Kenko Pro 1.4. I cannot see a lot of difference in IQ, maybe the Canon just shades it but as the Kenko can be bought for around a 1/3rd of the price of the Canon I would say that it is the 'best bang for the buck'.

if this helps any...

I shot an airshow with a kenko (midgrade) 2x TC on a 70-200 f/4L, mostly at 400mm

Compared shots with a guy from same airshow on a 100-400L, and there was a very noticeable difference in sharpnes

Although, I have 2 TC's and use them, and I firmly believe they are useful, there IS a loss of IQ on lower end models.. Stick to canon's or high end kenko

I am using both Canon TC's on the EF 300L 2.8. I have no complaints to date. I even stacked them for this shot:

i still use a cheap kenko 1.5x t-con that I thought was just a quick/cheap solution when i was low on cash, but it's still working out wonderfully for what i've been using them on (70-200 2.8IS and 100-400). When I do pick up my first big prime, I'll definitely invest in the canon 1.4 and 2x t-cons as they are supposed to be truly the best....but cannot be used on every lens as the element sticks out.