Nothing more needs to be said. Everything is fluid and runs fine, but multitasking is pretty nonexistent. If you leave an app for more than about a minute it will reload. This is not specific to the 6+, but the 6 as well. I am sure we will see 2GB is the new phones, same as the iPad air 2.
If you have a new android phone and multi-task a lot, you will be pretty disappointed. I always feel like I am rushing to multi-task on the iPhone. If I leave an app, I move at superman speed to prevent the reload.