Is it good to get the magnet?

I don't think it is worth getting the magnet skill, sure it helps getting the EXP dots which may be a bit further away, but there are more useful skills to grab which will have an effect on your power or HP in the game.

You can always pick up magnets during gameplay to grab all of the dots which are currently on the field.

A good strategy is to stand close to a magnet until you are about to be overwhelmed, then scoot over to the magnet to grab all the EXP and hopefully get some good upgrades.

Totally worth it near the end when there are just gems lying around and you are crushing the mobs. Great way to get coins as the gems lets you obtain it when you have no more skills available.