I use the J&G Cyclewear Waterproof Breathable Helmet Cover and really like it. I don't put it on until the rain really starts to fall, but I love it. I use a rain jacket with no hood, and the rain drains from the helmet cover over the collar and down the outside my jacket in the back - not inside the collar and inside my shirt.

The breathable fabric makes a big difference. I don't wear anything under it unless it is below freezing, and stay very comfortable. My head gets very hot, so this is a big deal for me. Th breathable fabric makes it nice for the "transition temperatures" - when it is too warm for a waterproof layer, but too cold for nothing. For me, this is between about 50°F and 30°F. Above 50, I just get wet.

If I lost it, I would replace it in a heartbeat. Especially where we live. Even if the waterproof breathable was not an option, I would still use a helmet cover. I'm not a fan of getting the "runoff" from the top of my head into my mouth during a steady rain.