One of R2's breaking changes is that EPiServer.Core.IPageStore has been removed.
A consequence of this is that if your code is using IPageStore.GetPage(...) you will get "method not found" exception.
This is an issue if you're going to upgrade to R2 and is using the Demo Templates for R1. For example the episerver.blog.dll is using IPageStore and since we didn't ship the source code you better wait untill Demo templates for R2 is released, which is in "very near future".
So to summarize, before doing an upgrade to R2 (which was very smooth and easy with the new installer). Do a test upgrade on a copy of your site and check all functionallity, external modules etc. And read the breaking changes document, and the installtion/upgrade doc.
... also to clarify...
you can instead use datafactory.instance.getpage() or pagebase.getpage()
EPiServer world now upgraded to EPiServer CMS R2, and if you can read this the forum is also working...