IMO the overview on the Spring.NET website sums it up nicely:
Spring.NET provides comprehensive infrastructural support for
developing enterprise .NET applications. It allows you to remove
incidental complexity when using the base class libraries makes best
practices, such as test driven development, easy practices.
The practical advantages I found are:
- it guides me to building understandable, testable and maintainable software, by moving out almost all infrastructural code from my presentation and domain objects; I find that I can easily and reliably make changes to my code
- it is non-obtrusive; gives you all the freedom you need
More on the advantages and disadvantages I found in this answer.
As duffymo states in his answer, I would never be able to "roll my own" application framework that comes even close to Spring.NET. I would never ever build an enterprise application in .NET without the use of an application framework such as Spring.NET or Castle.
Incorporating Spring.NET in your (Brownfield?) application is most certainly doable. I'd advice you to do it step-by-step and not try to use everything that the framework has to offer at once. So point out your problems (such as undesired coupling) and leverage that part of the framework that helps out (e.g. IoC).