I'll admit it: I'm a secret stamp-collector. Especially limited edition series such as the US postal service's Pioneers of American Industrial Design, which has 12 important designers from the '30s to '50s. On sale in July. [USPS via Core77]
These are gorgeous, but I use so few stamps now that a book lasts more than a year (or till the next rate increase) and not being a true collector, I would not buy them and just save them (though I save cool cancelled ones). Bulk mailers use postage machines or preprinteds, so there is little chance that I can even them in my box already cancelled.