What is your personal experience with being a "Design Partner" for a vendor?  What was your motivation to become one? better pricing, influence the features, something else?

Having input into product design is a key component to getting some thing that you want. Working with a vendor allows someone to tailor the product to ones needs you get a better product than something right off the shelf. It is time well spent in one will see ROI

I found that the main reason to act as a design partner has been being able to influence the partner's team composition - able to work with the "A Team".  As a result, higher quality delivery and better fit solutions have been deployed.  The features developed quickly found their way into the product for others - which on one hand was great (no custom solution for us) but on the other hand had the potential to expose IP (solution that could have stayed a Trade Secret).  Due to this risk/benefit, the pros & cons need to be weighted.