So, Boots brand 17 have come out with their own magnetized polish. I picked up a bottle to try out, since I've never tried magnetized nails before. Colours available are teal, lilac, gunmetal and blue. I chose the latter.
The colour itself is a metallic, shimmery blue with flashes of purple. It looks really nice on it's own before you've even used the effect! The magnet is attached to the top of the lid, you just pull it off to use it (see below) which is good, because if it was a separate entity I would almost certainly lose it.
All you do is apply a thick layer (thin layers do not work, as I found) and hold the magnet over it, close but not touching. Maybe not too close as the polish likes to jump towards the magnet!
I'm sure you could create more interesting designs if you moved the magnet around a bit, but I just held it still. I messed up on my middle finger a little. The formula of the polish itself it super easy to apply, I only needed one (quite thick) coat.
I love the effect it creates, and it was so easy to do. It's available in boots for £5.99/10ml, which is a bit expensive for a 17 product, but I think it's worth it.