I found this shirt to be lightweight and comfortable even during some super humid days. While it is semi-fitted, I found that it wore loosely, which I prefer in the summer for the extra air circulation. The shirt fit me well and was not at all restrictive, either during barn chores or while riding. I also wore it while going running a few times, and it was quite comfortable and cool.
I really enjoy the fact that this shirt is tagless - there are absolutely no scratchy tags, which are the worst in the summer. The long front zipper (10 inches) makes sliding the shirt on and off easy, and you can do so while wearing a ponytail without having to redo the ponytail. I also like the fact that there is a zippered pocket on the right side. Since I'm right-handed, this is very convenient, and the pocket is large enough to hold some horse treats, keys, or a phone, though the fabric is so light that I wouldn't recommend weighing it down too much. When not in use, the pocket isn't visible.
This is one of the most breathable shirts that I've worn. If you examine the fabric closely, you'll see that it's actually textured, which I'd imagine is part of what helps to keep you cool while wearing it. It definitely wicks moisture - I wore it for between 1.5 and 4 hours at a time doing heavy physical activity on hot days, but never found that the fabric retained sweat or got heavy.
The Kerrits Venti Shortsleeve is well-constructed - despite numerous wears and washes it's held up well and still looks new. The stitching is strong and accurate, and the zippers still move smoothly. I'd definitely recommend it for summer riding, especially if you live in a humid climate. For the price ($49.00 on the Kerrits website), you'll get excellent use out of a very comfortable summer shirt. Plus, it comes with an adorable tiny package of "Kerrit" seeds that you can plant and then feed to your horse later on. You can't beat that!