I use just a simple cable lock I bought at the department store. I rarely leave my bike unattended for long or in "sketchy" areas. I just want to prevent anybody from grabbing a bike for a joyride. My bike cost under $1,000 so I don't think professional thieves are going to go to much trouble to get it. I also use the cable/combination lock when I put my bike on my car rack. I know a thief could probably take car rack and bike together, but I also think it's unlikely. So my approach is to prevent creating a convenient opportunity for a quick bike.