As others have said, depends on the content being unlocked.
If it's purely cosmetic, like skins (or mounts/transmog items in WoW) then I don't mind, even if the game itself is one you pay for.
If it's required to progress in the game, then no, I do not like. It's not so bad if the micro transaction merely speeds things up, but even then I don't typically like it, because most games in my experience make it progressively harder to advance.
One game I think has done it perfectly is the mobile game Plague Inc. Free to play, can pay a small amount to remove ads and allow for speed up of in game time, and you can also pay (also small amounts) to unlock content- but said content is pretty easy to unlock through just playing the game.