I have the DeSoto cool wings. I find them much cooler than bare skin because my skin is so dark (no matter how much sunscreen I use) that it heats up. White armcoolers are better! I actually have worn mine while working in the garden as well.

I also agree with Oak - they work better in a drier environment (particularly if you aren't actively moving like when gardening). I didn't like my coolwings all that much when I lived in NC which is very humid, but they work fantastic out here in the dry Oregon summers. I've never known them to get water logged though - they stay drier than bare skin, actually.

Plus, the cool wings are nice to have because when you get off the bike, you can take them off and just be in a sleeveless jersey as opposed to being in something with long sleeves.