To everyone who's been having these problems with Visual Studio and Xamarin Forms here's what worked for me, based on @Brendan Zagaeski fix.
The simplest way to reproduce the error is to save the XAML file, and the .CS will start to present intellisense errors.
1) In your solution folder (VS) Open
packages\Xamarin.Forms.18.104.22.16829\build\portable-win+net45+wp80+win81+wpa81+MonoAndroid10+MonoTouch10+Xamarin.iOS10\Xamarin.Forms.targets in the Visual Studio text editor.
That is opening Xamarin.Forms.targets file.
2) Replace the contents with this file: https://gist.github.com/victorarce/8f42ae855457275d1924
* You should probably backup the file just in case.
3) Save the file, close VS and reOpen the solution. !Important
And as simple as that, my intellisense errors went away. This fixes both issues, since they where related.
Note: Im using the latest Xamarin.Forms version 6529 and VS 2015 Update 1, but I think it might worth the shot trying to see if it works with previous versions.
If you want to take the long road, you can manually change your targets file step by step following @Brendan Zagaeski suggestions like I did:
Hope it helps