You have to love these kinds of studies. The latest coming out of IBM Research is a 345 user study into how productive it is to file your emails into folders. According to their
filings findings it took users an average of 17 seconds to find an email via searching compared to 58 seconds to find an email by folder. Filing emails also did not make the success of finding the intended email any more successful. They claim that users are spending 20+ minutes a day filing emails in a pointless exercise.
They also delve into labels and conversation threads like you find in Gmail. According to the study users resisted tagging emails though did not delve further into it. On the other hand they found that conversation threading (a sweet feature that RIM added in BlackBerry OS 6.0) is essentially a form of custom search that makes it easier to find emails. IBM Research seems to be all for conversation threading though they want Superthreading which groups multiple conversations together under a topic or project.