EAV "can" be very fast and efficient if you use it for single-table storage. But that then requires EXTRAORDINARY effort on the application side to handle virtually all of the data processing needs and even when you do that it is useful for a pretty narrow class of apps. There is at least one Web2.0 company doing this, and I have another client using EAV to very good effect as well in a system that uses EAV constructs for a few tables.

If you are going to do pure EAV storage and jump through the required hoops on the application side to manage the data, it is honestly a waste to spend the money for SQL Server. Use one of the many free storage systems available.