* What is your evaluation of the design? Very good. It seems everything is configurable if needed and works as expected out of the box.
Perhaps intermodule_holder should be the default holder specifier as there is an unpleasant potential gotcha otherwise? The docs do not state why not as far as I can see. An efficiency issue perhaps? Whilst not having a strong opinion, I prefer the policy-based configuration interface as is.
* What is your evaluation of the implementation? Whilst not having a strong opinion, I like the operator== as is.
* What is your evaluation of the documentation? Extremely clearly written and well thought out. I would like to see more explanation of the examples. I like the inclusion of the test code.
* What is your evaluation of the potential usefulness of the library? I can think of several situations where I could have gained from using the flyweight idiom and this library would have saved me time and effort. I expect to encounter more in the future.
* Did you try to use the library? With what compiler? No
* How much effort did you put into your evaluation? A glance? A quick reading? In-depth study? A quick read through the tutorial and the examples.
* Are you knowledgeable about the problem domain? I'm not an expert in design patterns but I've been using C++ for a 15 years now.
* Do you think the library should be accepted as a Boost library? Be sure to say this explicitly so that your other comments don't obscure your overall opinion. Definitely.