I really like the MVC "way" and have actually enjoyed learning ASP.NET MVC (I never liked ASP.NET Webforms but I didn't know why until now). The problem is I'm about to inherit a bunch of Webforms code and wondered if I'll be able to add new things to the codebase with MVC instead off using Webforms. I suppose it depends a lot on how authentication is set up and if the code is set up in a way that I can include some library or something and just use those function or if I'll have to recreate them or gasp duplicate them somewhere else. What should I worry about? Should I just stop messing with MVC and get working on Webforms or can I make it work together so that if I need to modify something from the Webforms codebase I can just remake it the MVC way as I move along?
Well, MVC is built on top of Asp.Net so they should be able to play well together. The question is if the data is structured in such a way that it would lend itself to using the MVC framework and ideology.
Also think about the maintainability of it -- and not just for yourself, but for others. It might be better to just stick to webforms depending on where the application state is at. If you have to start doing more work just to get the application to work, it might not be worth the hassle.
Scott Hanselman has a post that talks just a little about it in his blog. If you want to start going down the MVC route then go for it. I think it probably depends on how big the ASP.NET Webforms app is.
I would recommend attempting to only add new stuff using MVC and convert over the legacy webforms code as you can.
This is a pretty decent post of putting them both together in the same project: http://www.chadmyers.com/Blog/archive/2007/11/30/asp.net-webforms-and-mvc-in-the-same-project.aspx