Marc Edwards at Bjango has a very cool article explaining how he uses Soulver in his design process:
There’s nothing wrong with shuffling and resizing layers around until reaching the desired outcome, but I often get quicker results by breaking out a calculator. And for this kind of work, Soulver is my favourite, because of the way it handles constants and multiline equations (disclaimer: Acqualia have no affiliation with Bjango, I just like their app). It’s a nice mid-point between a physical calculator (remember those?) and a spreadsheet, often trumping both for speed for designer-y calculations.
First, that’s awesome. Second, even if this isn’t up your alley it’s a clear indicator of how great Soulver is.