Do people use it for melee or just for its +PL? Which class/multi-class make best use of it?

For melee mostly. It's very good because of its various enchantments. One of the best two handed weapons in the game. And it has reach...

Class doesn't matter much. As long as you have melee prowess it's a good pick. Anything that hits fast profits more (at least when it comes to fun) from the 5 consecutive hits enchantments. Iirc Swift Flurry chains do proc that enchantment, too (if it's like 5 crits in a row I mean).