Well, trying to buy in-game via Origin (on the PC), doesn't actually let you buy it. There's apparently a glitch with their setup that keeps you from purchasing new BioWare points through the DLC interface, so...

Plus, I've heard that Leviathan introduces some glitches to the Extended Cut ending of the game, voiding out some of the new dialogue.

At this point, I don't really care, though. They may have made the ending a little more palatable, but it still tore away a lot that I loved about the series. If and when I sit down to play it again, I'll probably purchase it. Otherwise, oh well.