I've done this before for a product that we were trying to migrate from Web Forms to MVC and found it worked pretty well. Some notes from my experience:
I used the technique in this blog post to use our existing master page in Razor views. I wanted to avoid having to maintain two layouts, however in hindsight this was a mistake and maintaining two layouts wouldn't have been a better solution. The issue was that Web Forms concepts kept on "leaking" into our MVC pages, for example our master page used a script manager, which caused complications when trying to do thing the "MVC way".
We used a Web-Forms specific UI library, which meant that some UI elements had a slightly inconsistent look and feel.
I seem to recall we had some sort of issue with cookies where the Owin and Web Form APIs didn't interact quite as expected. I'm sorry, I can't remember the details, only that it stumped us for a day or so.
Its worth noting that we never managed to get rid of Web Forms - we just didn't have the time for a complete re-write. I'm not sure this was the best approach, but its also not a decision that I regret.