I have three of the 17 polishes, in Grey, Blue and Purple. There is another one, called Teal, but Boots are having 3 for 2 on 17, so I didn't get the fourth. These polishes retail for £5.99 in Boots stores and online.
As you can see, the large cap comes off to reveal a smaller one, which I find a lot easier to use. On the top of the cap is a magnet.
Grey is a graphite colour. It reminds me of pencil lead from the school pencils. Applies the sheerest of all three. You definitely need to put two coats on. Has black lines when the magnet is used.
Purple this is a pearly purple. Very dupeable, in fact I think that 17 have a colour identical to this, and I know that ASDA have a nail polish this colour, incidentally called Purple. Has black lines when magnet is used.
Blue is my favourite colour, when applied by itself without a magnet. It's a nice medium blue, with a pink pearl. I tried to get a photo of the pearl, but it was extremely hard, so have this. Has darker blue lines when the magnet is applied.
How this works is that you apply one coat of the nail polish to your nail, let it dry, then paint on another coat, and while it's still wet, you turn the cap upside down so the magnet is facing your nail and hover the cap over your nail. There is a little lip so you can rest it on your actual finger just before the nail without the magnet hitting the nail. If you do hit the nail, then you'll be left with a blemish. I just paint straight over that again.
Leave the magnet in place for around 10 seconds, and you'll have your magnetic effect!
Grey, Purple, Blue
The effect in real life is almost 3D. Tilting your nails will make the lines appear to move. I have had far too much fun with this polish, they dry really fast, look great, and are a lot more cheaper than their counterparts (Nails Inc). I would definitely recommend these polishes; even if you don't like the magnetic effect, the colours are good solid colours and the formula is lovely to work with.
You do have to play around with the magnet to see what effect you like, and you're not stuck with just the effect in the photos above. You can rotate the magnet to get different effects.
Available from Boots stores around the country or boots.com