I feel the Intel only models are not that good; their graphics chips seem to stutter and lag a lot on even when opening a stack or changing spaces. They won't be capable of anything heavy duty either. I have a 2013 15" with Nvidia and a friend of mine has a 2014 15" with Nvidia (the CPUs and Intel graphics on those is similar if not identical to the 2015 15" models), and you can definitely see a noticeable increase in performance when you switch between graphics chips (much smoother and a lot less lag).
I did find that to be the case with Yosemite on my Intel Iris mid-2014 15", but since El Capitan, it's been buttery smooth. Like @turbineseaplane, I too would notice if it gets laggy. Maybe it stays smooth because I don't have any graphically intensive apps open though.