Any experiences anyone can share about this tool?
It's a good time saver. They've solved some problems that you may have to solve yourself if you choose not to use them. Add-in problems as you're probably aware, can be fist pounding at best.
I used Add-in Express for a little while, but ended up going with my own add-in code based on the COM shim stuff. I needed more information as to what was going on during the registration process (because it was failing in a highly secure environment). That was the only reason why I switched though. Was pretty happy with the product and support. Just needed to be "closer to the metal", as the kids say these days.