Sure, Leonardo was a lovely guy, great technique, cutting edge, a bit flaky - but he was a Quattrocento Florentine, and (if you read Jacob Burckhardt only a little bit too literally) they were pretty much all like that back then. So what is the modern-day 'cult of personality' surrounding Leonardo really about?
An old friend's Italian partner once told me that people in Italy generally rated Brunelleschi over Leonardo: and I can quite see (Brunelleschi's famous sinking barge aside) why that might well be true. For me, there are two raw types of genius: visionary (who can see how things ought to work with a clarity the rest of us don't have access to) and practical (who make the impossible actually happen). Sure, Leonardo was a visionary genius, who managed to 'ship a few products': but Brunelleschi's genius comes across as both visionary and practical.
And so it seems to me that sometime over the last century, we (as a society) began to value the visionary over the practical (and the inspiration over the perspiration), as if we can somehow subsist on ideas without action. The cult of Leonardo merely rides this cultural wave, not unlike a carved figurehead on the prow of the ship we're sailing in: he was simply a good match for the impractical historical non-hero archetype we happened to be looking for.
Which is not to say that I don't value all the wonderful books on Leonardo out there: my two current favourites are the epic 3d model-fest "Leonardo's Machines" by Mario Taddei and Edoardo Zanon (Giunti, 2005), and Martin Kemp's splendid "Leonardo da Vinci: Experience, Experiment and Design" (V&A Publications, 2006). But rather, I see Leonardo as being the poster-child for modern anti-practical sentiments, chosen centuries after his death: and the modern worshipping of his life and work as being part of an ideological programme I don't really understand. The culture preceded the cult, if you like.
I can't also help wondering if the study of Leonardo is somehow holding back our notion of early modern history, as if we cannot but help look at the Quattrocento through the knotted cluster of ideas about invention we project so strongly onto da Vinci. Perhaps we can do better...
Anyway, today's gratuitous Leonardo link comes courtesy of The Guardian: a story about film director Peter Greenaway quite literally projecting his own story onto the Last Supper. Having said that, Leonardo would probably have approved: his career in Milan revolved not around painting or engineering, but around designing dramatic entertainments for the Sforza court and its visitors - he was essentially a film director without film.
Incidentally, I recall a Philip K. Dick short story where a whole sequence of "Mona Lisa"s are discovered, along with a huge wooden machine in a cave for "playing" them, like a gigantic zoetrope: which then reveals the (surprisingly saucy) secret behind her smile... But perhaps I just dreamt it. :-)