The speed of execution, ability to parse expressions quickly without having to write your own parsers from scratch or use existing frameworks that require you to rewrite everything in C#/VB etc., which can be very time consuming especially if it's not something you're familiar with already.
I also like how easy it was to get started using this library - just add an assembly reference in Visual Studio (or install via NuGet) and start writing code! Nothing really at all as far as any issues go but perhaps more documentation would help people who are new to parsing math equations easily understand what features they have available when working within their applications. Parsing large amounts of data faster than other libraries out there. We were able to do things much quicker than we previously could because of the increased performance gained by using our custom built class instead of one provided by another framework.