Priority doesn't really matter *too* much since M. Medicham is supposed to be a wallbreaker, it's comparatively slow and frail so it's easily revenge killed even without priority.
The main problem for me is that it faces direct competition as a slow fighting-type wallbreaker with M. Heracross, who is a little slower but is much bulkier, is similarly powerful, and has arguably better coverage (M. Hera beats Aegislash and doesn't beat the ground/flyings except it kinda beats Landog, Medicham beats the ground/flyings but not Aegislash: Aegislash is more common than the ground/flyings put together). M. Heracross's fighting stab is also far more reliable than M. Medicham's. I can't stand High Jump Kick, I'm sure I'm not the only person who feels like that.
Pretty much every time I've thought 'hmm, Mega Medicham' the slot has been filled better by M. Hera. I'm a total M. Hera fanboy tho so I dunno how much of that is just bias, I imagine the 100 base speed comes in useful quite often? And yeah I guess Medicham has priority but it seems kinda garbage, unstab unboosted Bullet Punch? barf.
I just tested my Mega Medicham (nicknamed Medi-Champ) against my friend who hadn't really seen it before. This is my set:
Medicham @ Mega
Adamant: 252 Atk/252 Spe/ 4 HP
~Bulk Up ~Drain Punch ~Ice Punch ~Bullet Punch
BP for priority, Drain for recovery and STAB, Ice Punch for Flying and general coverage, and bulk up for the attack boost and some pseudo-longevity combined with Drain.
It beat his Mega Scizor, but it's defenses ARe pretty frail. That in conjunction with its type-combo probably is why it doesn't see much use. I like it though.
I'd recommend using it on a Dual Screens team for better results. Too bad it can't learn SD.
Sounds like a less bulky, but slightly harder hitting (after Life Orb) and significantly faster Conkeldurr with a non-stab priority, that can at least scare Gengar from switching in and sub-disabling thanks to Bullet Punch.
Yes, it does get walled by several common things, but that's were the rest of the team comes in. Is a cool revenge killer since the Fake Out + BP combo does reasonable damage if unresisted. He's also quite fast (100 Base speed is the same as the Charizards). And I just love to blow Gengars on the switch, people can be very predictable in that.
Mega Medicham has an amazing combination of mediocre speed and terrible bulk, and it's only priority moves (Fake Out and Bullet Punch) are very weak. So it's far from invincible. It's great, specially if you can do some smart switching, but it can be taken down by a lot of things.
But yeah, people should use it more. If you are good with switching and prediction, this thing can put A LOT or pressure on the enemy team.