We built a Shipping SDK using SF in both the UI layer and in the Business Layer to perform all of the functions necessary to add UPS,DHL, Fedex, USPS into any .Net enabled Application. It can print labels, get rates, rate shop, and adjust for internal freight rules using one object model instead of customizing for each API from each carrier. This application has already been used to insert plugins to 3rd party products, Add Freight Shopping to Websites and we have yet to encounter a single problem with SF and it has made our data layer very very stable which I believe is the core of any Database related product.
Take a look at a sample video to see what we were able to do with SF. http://www.kellersystems.com/ISSI/ISSI%20Plugin.html" target=_"blank" class="SmlLinks">http://www.kellersystems.com/ISSI/ISSI%20Plugin.html