The company line: "When you're reaching way back to launch the 12 weight or polling across a hard, slick flat you need a shirt that moves with you. You also need cool, lightweight fabric and serious sun protection. This ultra-light (7.2 oz.) long sleeved shirt is made from quick-drying 2.25 oz. nylon polyester blend with mechanical stretch and 30 UPF sun protection. The vented upper torso catches the lightest breeze and vertical-zip chest pockets hold anything you need to access quickly."
Fit among manufacturers seems to be as unique as every body type. I'm a slim 5'11" and around 160 pounds, so I'm always annoyed when I order a shirt in a mens size M, put it on and find I could fit two of me in it (looking at you Columbia).
I'm particularly pleased that Patagonia has it right for me and not some grossly oversized version of a men's medium. Though they describe the fit as "relaxed," the fit for me is excellent: trim, but not at all confining or restrictive.
Other elements this shirt gets right: as described, I can verify that it is light. It does indeed dry quickly and sports a 30-UPF sun protection claim. I try to use sunscreen religiously even in long sleeves, so I cannot verify that this thing will protect you beyond its claim if you forget to lather up.
It has hidden roll-up buttons on the sleeves when you're either indoors and warm, ready for some sun or additional cooling. Most importantly for me this shirt has two side-zip front chest pockets. Ever since I acquired a shirt that featured these, any top-zip (or worse, Velcro) pockets have become real pet peeves and may become deal-breakers.
Stylistically, it's yet another classic Western-ish plaid. Yes, I own more than a couple of plaid shirts so I'm guilty of perpetuating the style. Suffice it to say I really do like the looks. And to Patagonia's credit, the branding is wonderfully subtle here: a color-consistent stitched label in the middle of the shoulders (dark green on this "Dunmore Plaid, Spanish Moss" version) that not only identifies it by the classic lower-case brand, but also a stitched "S-loop" above indicating it's part of the fly fishing line.
I really like hidden backside collar buttons to hold collars down, which this shirt lacks. Some more substantial mesh venting in the back/shoulders wouldn't go unappreciated either.
I've always been somewhat reluctant to splurge on Patagonia gear because it sometimes feels mildly overpriced. But, I can say with confidence that given the quality of the Patagonia Sun Stretch shirt and the number of things Patagonia has gotten right with this shirt, it will see enough warm weather wear to entirely justify the $99 retail.