We always try to think about the person that is going to ultimately use our SDK but the reality that is that we not always end up receiving all the feedback that we should. When someone is just starting to use our SDK, they do ask us questions (through our newsgroups) or they use our ISV program, but once they are up to speed, I've never heard their feedback or questions. I was personally responsible for the design of the UI AddIns for SBA and I tried to make a very straight forward API that used XML (so it can be extended in the future) and a universal method called DoAction, to call the Addin back after the user did an action. I don't know if there are any solutions out there that use them or not.
So far, we have a directory that lists the different SBA solutions from ISVs that are part of our program (the directory we have is here). If you are indeed developing using our SDK, please let us know what you think and what we did wrong.