Swimming as a form of exercise...?

I usually run for exercise, but I am thinking of incorporating swimming several times a week into my workout schedule. For anyone who swims, do you find that swimming is a good form of exercise--for both muscle definition and a calorie-burning workout? Also, for how long do you swim, and how should I feel while swimming (out of breath, or should I swim at a medium to slow pace) Any general swimming techniques, tips, and personal experiences would be helpful! And I want to know why swimming is not generally recommended as an exercise to prevent osteoporosis.