This is a key issue. So to a certain extent, it really depends on which lens you are going to use the TC on. Checking for compatibility is a must.Some, like the Nikkor TC-XXe series won't mount on a non-AF-S lens.
I think most will agree with this. I sure do. The Kenko worked well on my 80-200 f/2.8 (two ring), but even so, there was a very slight loss of IQ when shooting wide open. Not bad, but noticeable when pixel peeping.The best generis TC currently available seems to be the Kenko Pro 300 1.4.