Just letting people know that I've successfully upgraded the RAM on my mid 2012 MacBook Pro - from 8Gb (2 of 4 Gb Hynix sticks fitted ex the factory) to 16Gb (2 of GSkill 8Gb sticks) of DDR3 1600 RAM. It was easy to do, and worked instantly. It even boots up faster but I haven't used heavy applications yet. I sense already that they will run a lot faster.
A little while ago, I replaced the original spinning hard-drive (which was operating too slowly for my liking) with a 500Gb Samsung 850, and that worked perfectly, too.
Full of enthusiasm for the above, we've just replaced another original drive from a friend's mid 2012 MacBook Pro, with a 1Tb Samsung 850 and that worked perfectly, too. The whole job took 10 minutes and made a HUGE difference to the performance cf. the old 750Gb spinning drive.
Just wanted owners to know, because you can feel really good about owning these laptops knowing that upgrades with quality components should be very easy to do and should work really well, extending the life of the laptop and giving better performance from the next boot-up.