In particular, the new Nexus 7.

I'm talking about the ones that wake/sleep the device using an inbuilt magnet in the cover.

I've heard of them causing marks on screens, messing up compass apps, NFC etc.

Fairly sure none of this is true, but does anyone know for sure?

I very much doubt they can damage the screen. The magnets could mess up a compass (never tried it, but a field will interfere with the earth's field), but not permanently. They're also pretty weak magnets so would depend where the compass sensor is in the device and how close they were.

Can't see it affecting NFC either, but again not a permanent thing and depends on proximity to the tag or sensor.

Just don't buy a cheap one.

I have had a leather 'smart' cover on my iPad for 12 months now. Not a blemish on the screen.

Just be careful when you close the cover that you haven't got anything abrasive, grains of sand etc, between the cover and the screen. But that's just common sense really.

LCD screens are not affected by magnetism in any way whatsoever, neither the liquid crystal material itself nor the latching thin-film transistor (TFT) for each pixel.

They shouldn't but they can effect your memory cards if you have a device that uses them, and depending on what case you have there are reports of the magnets wiping credit cards etc if you carry them in the little pockets.

I thought memory cards were more like solid state drives now, not magnetic storage like floppy disks, that were severely affected by magnets...

eg http://www.photographybay.com/2010/0...ds-vs-magnets/

http://www.techhive.com/article/116572/article.html

^^ not sure, but I was looking for a case for my Z1 and reading Amazon reviews, quite a few on there complain about memory card and CC's getting corrupted.