If you have soft water and you are using it for dark beers, it works all right, in my experience. But if you are using hard water already, or you are trying to acidify your mash, 5.2 will not work. IMO, it's no easier (and less effective) than just doing the math with Bru'n Water and building your water from scratch.
What James says makes sense to me, as far as partial mashes are concerned, though I suspect Bru'n Water could still help you figure out what you need to do there (presumably adding acid or chalk to the mash).