Anyone ever tried extention tubes on a 100-400?

For macro, if so, how did it turn out?

The relatively long minimum focus distance and low native magnification make it difficult to get useful magnification from tubes....about 1/3 life size with 25mm behind it.
A better option would probably be a 77mm 500D diopter filter. This will get you close to life size at the long end of the zoom range.

Bob

I haven't used extension tubes with the 100-400 for true macro. I have that combination for dragonflies and butterflies (for working distance) and also for frogs. It works well for those types of shots but not for true macro.

I've had very good results with extension tubes on the 100-400. The thing to be aware of is that zooming makes more difference to the focus distance than focusing does. It sounds odd, but it's true, because the effect of the extension is directly dependent on focal length. Therefore, as you zoom out to shorter focal lengths, the focusing distance comes inwards, so you get a closer shot at shorter focal lengths than you do at longer ones. This actually makes for a very versatile setup once you start to get the hang of it.