Back in 2005 I wrote several posts addressing challenges related to building and packaging Office add-ins with VSTO. We managed to complete a few very cool add-ins for customers, but I have to admit that we did have our share of problems.
There are still developers out there who are faced with the same challenges that contact me regarding this topic. Unfortunately some of the related content for my posts are lost, and the original sourcecode is not available for the public (customer property).
Since then I've created a couple of Office add-ins, and at Puzzlepart we are currently using some prototype office add-ins to support our products for SharePoint 2010. For the past few years we've moved from pure VSTO on to using Add-In Express. Our experiences so far are very good, and I would like to recommend anyone that are about to write and deploy a Microsoft Office add-in to at least research the offerings from Add-In Express. Creating add-ins are almost always more complicated than you first think, and if this is not your primary development focus I strongly recommend paying for a license and getting the full value of someone with years of experience in this field to support you through a product and through their forums and support services.
Add-In Express are supporting all major versions of Microsoft Office, including Microsoft Office 2010 in both 32 and 64 bit versions.
Good luck creating great add-ins for Office!