Good - they keep my hair from getting messed up when I drive with the windows down. They let you drive with the windows down in the rain. They can really add to the appearance of some vehicles especially those with black trim.
Bad - If possible, hold them up to the window before you have them put on because they can be annoying to your view on some models. I had them on a Nissan truck and I never got used to them being in my line of sight at times. I have them on my Toyota Tacoma Double Cab now and never notice them at all. Also, DO NOT get those extra wide versions, I think they call them Bubble Shades, talk about blocking your view!!!
You can purchase and have them put on at any vehicle accessory dealership like Ziebart or Leonards. Look under Truck Accessories in the Yellow Pages. They'll cost you about $50 there, but you can be sure they're put on properly.