I like the gel, it's easy to apply, if you want better control, use an eyeliner brush to apply.
I know that some people left it on overnight, I tried that and didn't like the final tint that much, it's too much red in it. I left it on for 1hr following the packaging instruction and it turned out more natural, still has some warm tone to it though. I chose "gray brown" and I wish it had more ash tone to the color.
Other than that, I like the gel, it lasts me about 3-4 days, and I double cleansed every night.