You can manipulte Excels objects with code using the VBE. If you're wanting
to create a truly secure COM add-in then VSTO is the way to go. If you're
only wanting to create a .xla add-in you can use the VBE that accompinies
excel.
There may be some projects that would be easier with VSTO, but having used
VS.Net for a desktop app, I wonder if it might actually be harder to use for
basic apps. Without knowing what you want to accomplish, it is hard to advise
whether VSTO is worth considering for your specific needs.