Is using Db4o as a backend datastore for a Web site (ASP.NET MVC) a judicious choice as an alternative to MS SQL Server ?
The main issue with DB4o is: Can you cut your object net in some useful manner? If not, then you'll keep too many objects in RAM for too long and your performance will suffer.
For example, in SQL, you can create a cursor and then easily traverse a huge set of results. You can also query for a small set of columns while DB4o always loads the whole objects (and its references and the references of the references). With DB4o, you must make sure that DB4o doesn't try to pull in all objects from the DB at once.
You'll also need to get used to querying things your "DB" by filling out example objects which feels weird in the beginning.