A few years ago I got an awful case of sun poisoning while on holiday in Aruba.  Not only is the sun scorching, but swimming in the ocean strips sunscreen away with remarkable ease.  Being fair skinned as I am, the sun did not treat me kindly and my vacation was spent in a lot of pain.  Recently I had the opportunity to travel to the Caribbean again, but I purchased one of these beforehand.  Even without sunscreen, boating, swimming, and beaching all day, I didn't have a touch of sunburn under this guy.  As for comfort, there is an initial period that you'll need to get used to it if you've never worn something like this, but you can easily wear it around the house for an evening and get that over with.  In the sun it stays fairly cool and dries fast and you can almost forget that it's there at times.

I hear people complaining about the tightness of the shirt, but that's really how it's supposed to be.  Obviously you don't want the thing to squeeze you like a sausage casing, but it's not terribly effective if it's hanging loose.  Fit can be tough, but as a reference, I am 6 feet tall, 205 lbs, neck 16", sleeve 34/35 and the X-Large fits me perfectly.