I've had Iso-100 cinnamon bun. As John mentioned, the flavor is good, but you can't use very much milk/water if you want a nice consistency and solid flavor. It gets watered down pretty fast as you add liquid.
If you want a strong, cinnamon/buttery flavor that still tastes good even with a decent amount of milk/water, I'd second PES Select Snickerdoodle. It's arguably the perfect protein powder, with minimal fat/carbs, and an ideal ratio of whey/casein.
Another one I'll throw out there even though it doesn't get as much love on these forums is Trutein Cinnabun. The flavor isn't quite as strong as Select, but, imho, it's more of an authentic cinnabun flavor (if that's specifically what you're going for), with a notable "dough" element in the flavor profile. Between the two, on quality alone I'd choose Select, but this is a good one to consider as well.