The one I have has a 600kg pull force, but I would like a stronger one. Obviously you have to be able to pull whatever gets attached to it off, which sometimes needs a good yank. It's not particularly heavy, maybe 1kg. Usually I just stick it in a bucket attached to the rope, but I keep it in the polystyrene and box it came in when it's at home or if I'm travelling far. The only danger having it in the car is it sticking to it, but that's easily avoided. Takes a good yank to get it off if it does get stuck to a piece of steel as large as a car body though.

Electronics are never really a concern, the only thing the magnet is likely to come close to is my phone but I try to remember to not have it in my pockets while magnet fishing, and that's really because I don't want to drop it in the water. Fixed magnets have to move over a conductor to induce a current, and it would be quite small I think. The electomagnet in that breaking bad episode is very large, probably with a pull force magnitudes higher than a fishing magnet, and I can't speak for the shows accuracy (I don't actually remember what they used it for now).