<p>Which C#/.NET Dependency Injection frameworks are worth looking into?
And what can you say about their complexity and speed.</p>

Ninject is great. It seems really fast, but I haven't done any comparisons. I know Nate, the author, did some comparisons between Ninject and other DI frameworks and is looking for more ways to improve the speed of Ninject.
I've heard lots of people I respect say good things about StructureMap and CastleWindsor. Those, in my mind, are the big three to look at right now.