Generally I don’t use many Smart Collections in Adobe Lightroom. I do have a habit of flagging a few favourite images with a particular colour flag, and I have a Smart Collection to collate them together, and of course one for my five star images.
Most of my collections are project based or collections of published work or projects.
Reading the Lightroom Killer Tips website the other day, there was a great tip about creating a collection to hold your Smart Previews which I thought was a excellent idea.
It makes it far easier to manage your Smart Previous and get rid of the ones I no longer I need.