I received a Lee Filter Big Stopper for Christmas and I've been playing with it over the last few foggy days at the sea. I love how it makes you focus on composition and creating a simple image. You want to make sure you get a good shot when you are waiting for a 10 minute exposure!
This is quite a contrast to how I usually shoot -- fast a furious during magic hour & sunsets. I think you can gain a lot by slowing down and focusing on shapes. Also, it's nice to still be able to shoot when it's foggy and grey :)