I always wear a base layer, but I have many different kinds. Some are thicker for warmth, some are very thin (or mesh). They are all very tight against the skin and made of materials designed to wick sweat away. Some people in humid climates don't think base layers work for them, but it's relatively dry where I live and no matter how hot it is, there is a base layer that will work for me that is better than just wearing a jersey.

Edit: and echoing what Eric said, full zip jerseys are preferred. I often like to unzip on a climb and then zip it back up over the top. Of course it helps to be able to ride no-handed for a bit.