Magnetic mounts (mag mounts) are a very popular way of mounting a mobile antenna - because it is so easy.
So long as your vehicle is made of metal (ie not a fibreglass cab or aluminium like a Landrover) all you have to do is stick the mag mount on (preferably in the middle of the roof), screw the antenna in, plug the coax into your radio and away you go!
Mag mounts allow you to adjust the position of your antenna for best performance - this is usually as high and as central as possible.
The main disadvantage of mag mounts is that because they re outside in all weathers, and the coax connection is outside rather than under your roof, they do rust up and get bad connections inside.
You must make sure you choose a mag mount large enough for your antenna - if you do then you will have no problems, but too large an antenna (or too small a mag) and it will come off as you travel down the road.