I have been using Add-in Express for Microsoft Office and .NET for the last four years or so for all my clients' MS Excek solutions. It's an excellent Rapid Application Development tool (RAD tool) and provides a reliable and flexible development framework for my work. The RAD tool allows us, among other things, to work with the classic Commandbar and the Ribbob UI easily and where the outcome is highly professional UI. For Excel solutions I have used it to create managed COM, managed Automation, managed XLLs add-ins and RTD servers. It's also possible to create Smart Tags but I have no experience to do it. The tool can also easily create setup packages (MSI or ClickOnce) which in turn makes the deployment of the solutions smooth. I have extensively used their technical support, especially the e-mail support. So far I have only positive words about their support. They also offer a public forum for their clients and for their potential buyers. The technical documentation is good while the guide documentation lack more advanced examples of solutions. All in all, a highly recommended RAD tool!