When it debuted in 1989, it was the tallest and fastest coaster in existence and it was nothing like anything seen before. It started the new generation of 'Hyper Coasters', and started it off with a bang, too--no other hypers compare. It is currently the #3 coaster in the world and is one of my favorites as well. The main appeal of Magnum is all the bunny hills and the loads of airtime they produce.
The line for this is fairly short for the first couple hours after opening, but then in the early afternoon it will get more crowded and generally stay that way until close. I'd recommend riding it after Millennium Force or Dragster in the morning.
Magnum can be a little rough, especially in the very back, and though it is still fun in the back, I would recommend any seat in the first car most. There are 3 rows of seats in each car, and the smoothest ride will be in the middle seat, where it is not sitting on the wheels. My favorite seat is the third row of the first car as for some reason it seems to provide the most intense airtime.
The restraints can get uncomfortable on your legs or thighs during the ride, but if you keep yourself from putting your hands up and hold onto your restraint you may find it to be a more comfortable ride.
Magnum is certainly a must ride at Cedar Point, and I guarantee you'll enjoy the views on the first couple hills looking out at Lake Erie!