<p>Has anyone implemented a very large EAV or open schema style database in SQL Server?  I'm wondering if there are performance issues with this and how you were able to overcome those obstacles.</p>

I'm not an expert on EAV, but several more experienced developers than I have commented that Magento's open-source e-commerce framework is slow primarily because of the EAV architecture through MySQL. The most obvious disadvantage can't easily be overcome. That being the difficulty with which it is to troubleshoot where and how information is represented for entities and attribute values as the size of the application increases. The second argument against EAV I have heard is that it requires table joins that get into low double digits, but it was commented that using InnoDB over MyISAM improved the performance some (or it could be vice-versa, but I can't remember totally).