Disclosure: I sell a few sets of burrs here and there as part of a small side business.
I've tried Italian made aftermarket burrs and could not detect any difference in performance or in how they installed. But from my limited number of wholesale sources (I do not buy directly from Mazzer) there's almost no cost advantage for me in buying the aftermarket burrs and my customers don't seem to be interested in them.
Reliable sources tell me there are only two burr manufacturers in Italy who make the burrs for all the grinder companies. But there are burr manufacturers in Spain and a customer I spoke with in the UK mentioned having some quality issues (e.g. burrs not aligning and fitting perfectly when being mounted) with aftermarket burrs he had purchased there. My guess is that they were of Spanish origin.
That said.... when Mazzer recently changed the 33D SJ burrs to a 33M designation it appears that they changed suppliers . Packaging switched from cardboard blisterpack with formed plastic top to a plastic bag of sorts with the Mazzer branding on it. The real change, however, is that the 33M burrs appear to be a more precise casting and an overall more finely crafted appearance. They also have the word Mazzer printed on the flat side of each burr. Perhaps a way to deal with aftermarket products?
I haven't tried the 33M burrs yet and seriously doubt that I'd be able to tell the difference - even side by side - but they sure do look better. As for the "duranium" product - which is what the titanium alloy burrs are often called... again I have not personally tested them (but have sold about a dozen sets or so over the past year). There has been a fair amount of discussion at the home-barista.com forums on the topic and some folks like 'em - some not so much. There appears to be a small increase in grinding speed (i.e. how long it takes to grind a given weight of beans) and no solid indication of a qualitative difference in grind characteristics but - of interest to commercial users - a significantly longer burr life. Estimates range from 3X to 6X the usable life versus standard burrs but I have yet to come across any true A/B burr life comparisons done under controlled conditions.
New York Central Coffee Roasters