I needed a solution that allowed me to customize Excel files for my startup, a company that sells a Microsoft Excel training tutorial (picture a book written in Excel with interactive exercises). Each file needed to be processed specifically for each user, embedding their information, and then locking it. I spent a lot of time researching all of the options and Syncfusion was the only that was able to handle the complexity of my document (it uses lots of settings, functions, and formats). We ran into a few problems on the way and Synfusion support was super quick to help debug what was going on. I highly recommend Essential XlsIO.