I tried to use it a lot when it came out. It seemed like the strategy would be to use trial and error as a method for teasing out tricks in the syntax. However, very little seems standardized, and what works one month, means something else the next month, so it's difficult to get "good" at using it.
It is also difficult to be explicit about types, such as what you mean if you input "orange", or "Washington", and it doesn't seem to allow grouping with parentheses, etc—though admittedly I gave up pretty quickly when I got inconsistent results.
It probably did become a better "computational engine" than Google Calculator, once Google Calculator seemed to shift to a smaller focus on what it could do.
At one point I thought maybe its best functionality was as an alternative to Wikipedia, when comparing different members of a category, because the information would be consistently structured. Yet, the source data is pretty terrible. One, because they appear to have had little strategy for keeping it current, and, two, because without user input it's even more likely that the source is something bizarre, when with Wikipedia someone with domain specific knowledge would have been able to lend a hand.