James,
you ought to see them with the 85! My Kenko tubes cost about $170 for a set of 3. A friend was wondering what to get her daughter who is graduating, has Canon digital gear and really loves photography and loves to get close to things. We discussed the macro lens options then I suggested the extension rings as a low-cost option now so that if she really wanted to go whole hog with macro, she'd have the option of buying a dedicated lens later. She decided that was the right answer.

Clearly, you get sharper images overall with a dedicated macro lens. But when you combine a good, sharp prime or zoom (like the 70-200) with moderate tubes, you can get some really cool images. Here's one I took of my neighbor's Bird Of Paradise with the 70-200 and a 12mm tube.



I find that things like extension tubes and telextenders let you "extend" (pun intended) the range of your lenses and capture images that you would not be able to capture otherwise. A few months back, I tested the 100 macro against the 85 for portraits with the added benefit that it would do great macro. It's a wonderful, sharp lens. But I decided that I really needed the speed and OOF image quality of the 85. Now, when I use the 85 with tubes stopped down to F11, I find that it's a really sharp combination on its own.

BTW, I've blown both of those macro images up to 16X20 and display them in the studio. They look awesome.

Jim