Summary: I wasted about 5 months with a Kenko 1.4x The kenko had a high missed focus rate. Out of 10 images maybe 3 would be in focus w/ the kenko attached to a 70-200mm f/4. Moving to this teleconverter was the best thing, it's worth the 125.00 extra if you are on the fence. Also the gasket that extends into the barrel of your super telephoto helps w/ the weather resistance.
Summary: Used on my 70-200 f4 IS and Canon 40D, this converter has given very good results. The trick is to use it with the 70-200 zoomed all the way out to 200. Used this way, the images are very sharp.
Pros: Lends flexibility to existing compatible lenses. Lighter than getting a dedicated long telephoto lens, great image quality.
Cons: Image quality degrades when lens is not at full telephoto - no zooming. Pricey!
Summary: When attached to my 300mm f4 lens, the results are sharp and clean. Gives me that extra reach when I need it. I've used it in snowy conditions and have had no problems. The images are worthy of publication and thats what Im looking for.
Summary: I purchased the extender give me more range for my Canon 70-200/2.8 telephoto at soccer, softball, etc. games. The 1.4x entender now allows me to get the close ups I wanted over the majority of the playing field. Giving up one f-stop for the extra range is a good trade-off with my 40D.
Pros: Durable, Easy to Use, Improves Function, Lightweight
Summary: Bought to give my 70-200L IS some extra range to cover MMA bout. Got me closer to the action while still being fast in autofocusing. Will be a standard part of my bag when I cover future events.
Pros: Compact size, Easy to Use, Kept fast autofocus
Summary: I bought this extender to start doing some sports photography with the 70-200mm f/2.8 IS that I already own. The improved reach is a great help. I noticed that with brightly lit trees in the background of my shots that the bokeh (out of focus areas) were not nearly as nice and smooth as they were without the extender attached. The overall image quality was still very good and with any other lenses of smaller apertures it would probably be unnoticeable.