Well, your basic non-packer STP can be worn sort of sideways across your belly just below the band of your underwear, and be completely unnoticeable. Or you can tuck it vertically into the band if you're doing something athletic. Then you just wear a regular softpack in your briefs/jockstrap/whatever, which is perfectly comfortable.
An STP packer will always have the STP portion either between your legs or pressed up against your pubic area...I don't see myself being able to wear one comfortably. Some of them have softer STP attachments (like the nipple type) which wouldn't be as bad as hard plastic, but...meh, I'm happy enough just wearing a softpack and tucking my STP in separately. Separate STPs are easier to use one-handed anyway.
