As with rooting on Android, it used to be more popular, but in recent versions the balance between the effort required and benefit gained has tipped toward effort, so for most people it's just not worth it. (You'll need to wait for the next iOS version to be hacked before upgrading, which can leave you without security updates, new app features, and in some cases new versions of your favorite apps.) What's more, Apple is constantly and aggressively trying to stop the practice. The disadvantages are that it's difficult to do, it voids your warranty, it leaves you open to all kinds of new security threats, and it stops you from updating your iPhone as normal. Jailbreaking gives you a lot more freedom in terms of what you can do with your iPhone.
