It is the cheapest Sennheisers on the market, they will not be the best. There is also no guarantee of performance, which changes with e.g. difference in grade of steel out of China (which got really bad over the last few years).
Any burn-in period people talk of is usually their ears getting used to the sound, there is little to burn-in other than psychological. Speakers are different, they have large rubber surrounds that need to soften to allow the cone to have large, even travel.
I use the same model for live sound engineering and my hearing has a flat frequency response (except a null around 6.5kHz like everyone). They are good for budget earphones. A friend has $4000 custom made in-ears which sounds phenomenal and comes with a calibration certificate. You need to pick something sensible between the extremes.