I agree with you, this type of product selection (Ajax-interactive GUI) feature is great, and provides clues of how product category selection will be done in the futures price comparison engines.

You wrote;
” and the reactions were all over the place. Which is probably one of the reasons why the established shopping comparison engines stick with the lowest common denominator look and feel circa 1999.”

The point is: Why your friends’ reactions were all over the place?

Very little details in the GUI make a lot of difference in users cognition. In my opinion, one of the biggest issues in “Amazon’s Create Your Own Ring” is that you have to choose the ring category before seeing any result (for instance the category selection occupies the whole screen). You can see the number of available rings, but not the rings themselves.
In broadband times, users are used (and extremely eager) to see results for their search right away, or at least get immediate feedback once a category is chosen.

If Amazon had made the category selector smaller and provide immediate results (with pictures), the GUI would provoke a totally different cognition (thus a better perceived experience).

A similar example of interactive diamond selector (made in Flex) is:
http://d-p.com/ria/Diamond/index.html

GUI innovation can be a dangerous territory, but can also translate in serious business opportunities. So far most price comparison players (and even start-ups) have been ultra conservative in their GUI approach.
As you mentioned, only with experimentation we will move on from this “look and feel circa 1999″ price comparison experience.

Guilherme Leal