I've been looking into getting one, but they look like they'd be really uncomfortable while you're doing anything even slightly athletic.  I really wouldn't want to have to take it out every time I go do something.

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.

I'll weigh in on this one. In my experience, they are very comfortable. Feels super normal and not uncomfortable at all (I use DJ's). I do anything and everything in it--hiking, biking, chopping wood, swimming, you name it. If loving my STP is wrong, then I don't want to be right!