When did it happen and who did it?
Some time between 1469 - 1536, judging by the Erasmus mention below, and everyone from schoolboys to playwrights to politicians:

"Erasmus instructed them how to do it . . .The practice spread everywhere in early modern England, among ordinary readers as well as famous writers like Francis Bacon, Ben Jonson, John Milton, and John Locke. It involved a special way of taking in the printed word. Unlike modern readers, who follow the flow of a narrative from beginning to end, early modern Englishmen read in fits and starts and jumped from book to book. They broke texts into fragments and assembled them into new patterns by transcribing them in different sections of their notebooks. Then they reread the copies and rearranged the patterns while adding more excerpts. Reading and writing were therefore inseparable activities. They belonged to a continuous effort to make sense of things, for the world was full of signs: you could read your way through it; and by keeping an account of your readings, you made a book of your own, one stamped with your personality. . . .
Robert Darnton, "Extraordinary Commonplaces," The New York Review of Books, December 21, 2000"

Any educational value?
Looks like it, though in schools more than universities ...

"Boys ... had to keep notebooks or commonplace books in which to record, and then learn, idioms, quotations, or figures useful in composition or declamation. Not a little of that wide learning and impressive range of quotation adorning Elizabethan literature comes from these commonplace books." Schools in Tudor England, by Craig R. Thompson (Washington: Folger Shakespeare Library, 1958): p. 16, cf. 44.

Although the primacy of philosophy long remained unshaken in the universities, humanists gradually established the primacy of rhetoric in secondary education. They did so by means of a curriculum, the studia humanitatis, which chiefly comprised grammar, rhetoric, history, poetry, and moral philosophy.

This curriculum was inculcated by means of commonplace notebooks, the direct descendants of medieval florilegia. Students compiled these notebooks in the course of their readings in order to create a stock of ideas for their own speeches and compositions.
-- Michael E. Hobart and Zachary S. Schiffman. Information Ages: Literacy, Numeracy, and the Computer Revolution (1998) p.99

And if you haven't got it yet ...
I completely agree with Hanna

To me, this seems very similar to blogging: selection and transcription of important phrases, lines and passages from texts; addition of one's own comments and notes; provision of an index or other classification system so that important themes may be identified; creation of a work stamped with one's personality via the choice of passages and nature of comments and notes; rearrangement and alteration of emergent patterns through the addition of new excerpts.

Immediate responses
- Rhetoric? Did they say rhetoric?! Wey hey!
- What was the argument for not using commonplaces in University?
- Categories - did people copy each others? I'm thinking taxonomies and PCM here.
- Got to do more research on this, and probably brush off a whole load of cobwebs that are covering Mr Orated Sophistry himself, Protagoras and the topoi koinoi/loci communi.
- And there's a good starter bibliography here
- Definitely time for a cuppa and a think.

Sources:
- Thanks to Lilia's del.icio.us feed which led me to Jill Walker's blog which in turn led me to Hanna's original, and quite frankly bloody marvellous, post.]
- Good quotes from Commonplace book and Prof Lucia Knoles