It seems like it fills holes that Google has rather than acts as a complete superset of Google functionality. I use Google for a lot of the queries they demonstrate, but I have to click-through and dig further to find my answer. With Wolfram|Alpha, there is no need to dig. Yet, it doesn't return "regular" ranked results, which don't require computation -- hence, it's not a superset, and not a killer.
I can see Google responding with a similar offering, especially with their impending Squares release. It'll be interesting competition flaring up in this space.
I was tempted to agree there -- but that's the thing, whatever comes along and replaces Google won't look like Google, and we probably won't even notice it happening.
I don't think Wolfram Alpha is that. I think its goal is honestly too ... computational, logical, factual ... to have the broad appeal of Google. But I don't think that because something is fundamentally different than Google means that it's not the fabled Google killer.
I can see Google responding with a similar offering, especially with their impending Squares release. It'll be interesting competition flaring up in this space.