My daughter had a chance to use this over the weekend during a quick trip to Phoenix. We went for an early morning hike but it was already pushing 85 degrees by the time we were done and most of the hike was in full sun. The shirt is great for days like this - it's lightweight enough that she can have long sleeves without overheating too much.
I did find that I preferred the Patagonia Sun-Lite Hoodie to this since having the hood is a nice feature. The collar on this shirt does a pretty good job at protecting the back of her neck, but having a bit of hood to bunch up back there gives just that much more protection from the sun.
I got this in the 12 month size (which is actually 6-12 months on the tag). It's a little oversized for her (she's 17 pounds and 26 1/4" long right now), but having a looser fit is nice for warmer temps. Compared to winter items that we've gotten from Patagonia that were enormous, this ran much closer to "true to size."
The fabric is plenty soft to be straight against skin without needing to have a onesie or other layer underneath. The fabric has a decent bit of stretch to it as well.