I've just started working on a new cross platform library, and decided to try out the new multitargeting support that came about with the project format.
I based my work on Oren Novotny's "Multi-targeting the world: a single project to rule them all" and the GitHub project "XamarinCommunityToolkit". (The forum says I need to be around a bit longer to post links).
For the most part, it works well... in Visual Studio on Windows. It seems VS for Mac does not really understand it. From reading the log, I think it only builds the first TFM, and no intellisense works for iOS or Android code. I also get various odd compiler errors based on specific things I do (like when referencing an iOS library project, it says it can't find any project info).