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
I know this post is pretty old, but I though I would give my opinion about Sitecore MVC anyway. I've started working on a project a few months back using exclusively Sitecore MVC. There are a lot of restrictions to what I work with since this project must work with or without CMS and be able to fit in with as many CMS as possible (We currently use 2).
ASP.NET MVC was a no brainer for us. It is 2015 and we must go ahead with the new technologies. We are using Sitecore 8, and I think that Sitecore MVC has become mature with Sitecore 7.
There are still a few bumps on the road though. If you plan on using Sitecore with form posts, make sure those are made using AJAX. Doing a validation on a field can be tricky if you use regular POST actions, but there are workarounds.
Now there is Habitat project.
Sitecore Habitat is a Sitecore project that is bulit using the modular architecture. In their website they present a fully working example to install and test.
Habitat project:
https://github.com/Sitecore/Habitat
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.