I have used NS Formula since its inception in April 2011. I cannot fault it and cannot believe the power of the extension at the price I purchased it for.
Every update has been welcome and has included improvements that I didn't even think of, but glad were there.
With core math skills there is not much you cannot do with this extension.
Not only is it functionally A1, it was built by someone who has great design skills; so it looks sharp and calculates smoothly.
I used this extension in a simple "interest" website, but it and its updates have been so good that I am now releasing a full-blown business based on this extension.
End-users love it too; my traffic has grown on average at 30% a month since April 2011, and the whole site is based around NS Formula applications.
Overall Review: 5 Stars