Soulver 2.0
It’s quicker to use than a spreadsheet, and smarter and clearer than a traditional calculator.
So here is the problem for me: Soulver is not quicker than a traditional calculator.
Soulver is certainly better than Mac OS X's or the iPhone's built-in calculators. Try it out if you use these built-in applications or are a heavy user of Spotlight's calculator functionality.
I downloaded a trial of Soulver a while ago but I haven't found a use case for it for me. It's very neat though…
Update: To clearly bring my two points together, the reason I have not found a use case is that I prefer my Casio fx-570ES. I find its speed and convenience more useful that Soulver's greater clarity and smartness.