good timing, I just got my Kenko Pro300 DG teleconverter a week ago, specifically for my 70-300L on 7D.

I was planning to do a good review of this combo, but it's crap weather and I haven't had time, so here's the main points:
Firstly Kenko has 3 types, MC4, MC7, and Pro300. Older ones are marked 'DG', newer ones are 'DGX'. I'm not sure if that makes 6 permutations or less, doesn't matter. I'm speaking for the Pro300DG because that's what i've got.

On the instruction manual/sheet I got, it specifically says that 10-pin lenses (like 100-400L and the tele-primes), the aperture gets translated. For 7-pin lenses, the teleconverter is 'invisible'.

What that means is:
- your EXIF-data will show 70mm to 300mm, f/4 to f/5.6, even though you're in the range of 98mm to 420mm, f/5.6 to f/8.
- the camera thinks a max 300mm f/5.6 lens is attached, so it will try to AF, all the time. (if you attach, say, the 100-400 to a non-1-series, the camera will see that the aperture is f/8 and not even try to AF).
- But, it's very hit-and-miss with actually locking on. So far I haven't tried it in really good sunlight, but at 70mm (98mm f/5.6), it's not so bad, and can lock on easily enough (outised just then, in cloud). The longer you get, up to 300mm (420mm f/8), the worse it gets. I'm having a lot of trouble locking onto anything at 300mm (420mm), but then I haven't tried a proper test-target, nor in good sunlight. Sometimes it does nail it though, but it's going to take a lot of practise, i've been on AI servo so far, i'm going to try one-shot and Full-Time-Manual to fix it if it misses.
- Because the converter is 'invisible', the AFMA you set for the basic lens is the same as with the teleconverter. With the teleconverter attached, the camera still thinks I have a plain 70-300L attached, so when I've taken a photo, the EXIF still says it's used the same +2afma-value as I've set for the plain lens.
- I've tried using it with manual-focus primes (with no chip), and if i try take a photo, the camera gives an 'error 99' and says the contacts are dirty. An MF lens with an AF-chip on the adapter works fine though.

- But i'm still happy with it. I already had my 70-300L when i bought the t/c, so for $60 (ebay) I couldn't do any better. For the next-cheapest options for longer-lenses with AF, there's the 400 f/5.6, the 100-400 f/5.6, 200 f/2.8 and 2x t/c, or a 1-series body (and then I lose the 7D's 1.6x crop), all of which cost well over $1000 and then some. Even if I can only ever use it in MF, that's still cheaper and much better IQ than an older manual-focus 400mm lens, plus i get the great build-quality and IS of the 70-300L.

- But if you don't own it already, think about how much you need to get to 420mm. If you want to be there all the time, keep the 100-400L, the shot you take with AF will be better than the shot you miss with the 70-300L+tc hunting (not sure about IQ comparisons, although i'm guessing the 100-400L will be better anyway, natives generally always beat shorter+tc). Other than that, the 400 f/5.6 prime is cheap without IS, the 70-200 f/2.8L non-IS with a 2x t/c gives (i've heard), not as good IQ as the 100-400, but it's more versatile and faster on the wide end.

Meanwhile, the sun just came out, i'm going to see what it does in brighter conditions before the clouds come back...