Older Than They Think: The idea of Lex and Clark being friends in their childhoods has been around much longer than the TV series Smallville but new readers might believe this is a Canon Immigrant. However, Lex's dad does resemble Lionel Luthor from the series. Indeed, much of Birthright is designed around Reimagining the Artifact for older Silver Age Superman concepts, and the idea of Clark and Lex knowing each other at a young age is among them.
Vindicated by History: When it first came out, it was sharing space with the mediocre letdown of Superman: For Tomorrow and the notoriously awful Chuck Austen Action Comics run, both of which were more hyped up than it. Add in Leniel Yu's art tricking people into thinking the book would be yet another Darker and Edgier take, and it got lost in the shuffle - as Mark Waid himself put it, "I feel like I played Carnegie Hall and no one showed up." In the years since, as people discovered it on its own merits, it's become regarded as one of the best Superman stories, arguably his best origin, and the perfect Gateway Series.