Haven't had an aerial on the Panda yet, but have had aerials on a couple of Hatchback Focus's as i'm a Radio Ham.
Mag mounts aren't bad depending on what type you buy. If you do alot of motorway driving you want to make sure that it's going to be stong enough not to pull or slide loose (seen that happen before). This means either buying a Magmount with a very large magnetic surface or you can by them that have 3 magnets set onto a trangular Base.
Like this one:
http://www.haydon.info/images/69d.jpg
As has been mentioned you need to make sure that you place something between the mount and the car to protect it from scratches, on the magmount i used in the past i glued a felt pad on the underneath (same sort of felt that you seen on the bottom of ornaments to stop them damaging delicate surfaces.
The alernative that i tended to use instead was a Hatch Back mount, something like ebay item number 220473824096 (just put it into the search bar on ebay).
The advantage of these is that as long as there fitted correctly they won't slide off and are normally cheaper to buy than an equivalent mag mount.
The advantage of the mag mount over the boot mount is that as it normally sits in the middle of the roof it gives the aerial a better ground plan.
Hope this Helps
