From my own experiences...
Methylone (bk-MDMA) feels exactly like MDMA, but less intense in every respect, and shorter lived. There's not much more to describe the experience- it does not have the fiendish quality that mephedrone does, in fact, the wearing off point is oddly relaxing and pleasant in my experiences.
Mephedrone feels like what you'd expect cocaine and MDMA to feel like (when swallowed, if insufflated, it feels very speedy), it's extremely social and happy. I've never personally had an extreme desire to re-dose, but it lends itself to it. It's not visually distorting at all, and it's an ego strengthener (whereas MDMA is an ego softener). It's very much a party drug, and not something you will enjoy at home by yourself like you might with MDMA or bk-MDMA.
I personally grind my teeth a bit on both, but not to the extent I would on MDMA. The experience with both these chemicals is different person to person. I actually prefer them together- I start with mephedrone, and end with methylone, as it allows me to sleep and takes away any desire to re-dose.