Thick enough that they don't melt right away but still thinner than the "old school" ice pack I have from years ago. Have used with one or two other varieties to keep a small insulated cooler cold with snacks/picnic items in a hot car on 80+ degree days. No issues with leaking but have only used twice.
Just pop in the freezer. In the morning, I put one in the bottom of my insulated lunch tote (designed to fit this size block). Things that need to stay cold, like a container of chicken salad go right on top. It is still cold at lunch! Buy two to swap out, or to put on each side of your lunch bag to keep things extra cool! No need to find a fridge!