I think it's a pretty crap integration personally. It's a popup. Popup's are ugly and annoying often blocked by some antivirus, addblockers, etc. They could easily have integrated it more into the site with the videos more of a slide in feature. Don't get me wrong it's good it's added and will help both Google and Youtube in terms of traffic. I just think from a design aspect with the rest of G+ looking and working so well in design and UI (eg circles), I think they could have done better.