Cartridge burn-in

A pretty decent experiment would be to take one cartridge that has over 50 or over 100 hours on it, and then get another brand new cartridge (the exact same model), and play the two side-by-side on the same turntable, same tonearm, same setup, same phono, same preamp, same amp, same speakers, same room. That should tell us if there are audible differences between the cart that is broken in and the cart that is brand new out of the box.

To make that experiment more 'objective', the listening test between the two cartridges should be blindfold. The listeners must not know which cartridge is playing when. They are to make notes after listening to each cart, and then compare notes with the layout of the experiment.

Who's game?

No matter how you design your experiment, unless you are willing to spend money to do it, the design is fraught with sources for error that could skew the results.  If you compare a used or broken in cartridge to a new one, then stylus wear could be an issue.  If you set up the two cartridges in the same tonearm, then there can be error in set-up that could affect the results.  But the experiment is unnecessary in my opinion; it is very obvious and logical (given that the suspension is so important to performance) that break-in for a cartridge is real.  However, if you are skeptical, go for it.

I bought an Aidas Rainbow 3 months ago from @solypsa which sounded great on day one but now sounds truly magical. It seems like it has really opened up recently.

Glad the Aidas is continuing to satisfy :)

I've now installed about a dozen MC cartridges in my setup, from brand new. Some sound spectacular right from hour #1, and others start out decent-ish and then show improvement over the next 20 - 50 hours. I've even experienced both types from the same same brand. But I haven't ever had one sound "cringe", yet.

I don't think I've ever heard anything go from cringe to good via break-in. But to be fair, I lose patience easily and have given up on some components (not cartridges) before full burn-in. 

There is some logic in what @lewm says. Rubber has a short memory. Just over night the Porsche will develop flat spots on it's tires so for the first several miles the car rides rougher until the tires warm up and the flat spots resolve. I do not know if this is the case for the rubber used in a cartridge's suspension. The MSL Signature Platinum sounded great out of the box. If there is a change I do not hear it. Same is true on warm up. Esls for certain break in as the diaphragms loosen they become more dynamic and will produce better bass. Cartridges being electro-mechanical devices could have a break in period but I do not hear it. IMHO purely electronic gear does not break in. It is what it is right out of the box and first impressions are always the right impressions as the human mind can accommodate to almost anything.