StructureMap is the oldest, continuously used IoC/DI container for .Net dating back to its first public release and production usage all the way back in June 2004 on .Net 1.1. The current 3.* release represents 10+ years of lessons learned in the StructureMap and greater .Net community -- while also wiping away a great deal of legacy design decisions that no longer make sense today.
StructureMap is a feature rich IoC tool with support for interception, object lifecycles and intelligent disposal patterns, open generic types, modular registrations, conventional registration, custom policies, and all the injection pattern support you would expect in a modern .Net IoC container.
IoC tools can be configuration intensive and might fall into the "magical" tool category. That's okay though, because StructureMap has you covered with a lot of diagnostics and smart exception messages to help you solve whatever problems you might encounter.
StructureMap has been successfully integrated into most of the most commonly used .Net frameworks. StructureMap can also help you manage lifecycles and resource cleanup to allow teams to be more efficient in their usage of persistence tools like Entity Framework.