We are starting off a new project with sitecore as our CMS. I was thinking of using Sitecore as the Content Authoring Tool and use ASP.net MVC as in the Content delivery(CDA
As luck would have it, I'm currently working on two large projects using both technologies respectively. And while I'm big fan of both, I can't see any benefits to merging the two.
As far as Sitecore goes, there is a learning curve, but quite honestly in my case, since I actually learned ASP.NET MVC "first" as opposed to Web Forms, the learning curve has also been slightly attributed to some of my inexperience with Web Forms. That said, there still is most definitely a learning curve involved with Sitecore, but there are plenty of training and reference materials floating around to help with that. Also, the web controls that come with Sitecore make it feel a lot less like building a straight Web Forms app. Plus there's the option for using XSLT as a rendering engine which comes in handy as well.
If this is just one project you're thinking about, I would say just stick with Sitecore as it's presentation system is quite well thought out. And as Mark said above, it would really complicate things quite a bit I and I'm also not sure what there is to gain from it even. Also echoing the sentiments of commodore73, building stuff in Sitecore seriously does feel like you're using MVC already, just using a different framework.