MY EXPERIENCE
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I have purple hair, and they do not do purple clips to match it so these are the ones that I go with. The blonde colour ones and the black ones stick out like a sore thumb in my hair but these brown coloured ones seem to reflect my purple off them and blend in well with it.
I often put my hair back and I have A LOT of baby hair so I have to use 3-4 clips every time.

PRICE
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These are £1.49 from Boots for a packet of 30. Which is okay as they are good quality. - though if you are anything like me (I have stopped using the Boots ones) this is expensive due to how often you have to buy a new packet through losing them (OR BENDING THEM INTO WEIRD SHAPES WHEN I'M BORED). I have resorted to buying packs of 100 for £1 from the pound shop.

QUALITY
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Compared to my pound shop buys you can tell that the Boots ones are much better quality. The paint does not peel off on these and the bobbles on the end which make them safe do not pop off as quickly. They are also tighter so you don't have use as many.

SPEC
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They are about 4cm long and 1-2mm in width (metal separate to air). They come clipped to a piece of card that is very handy as you can keep them all together is you are organised, and although i'm not i'm sure plenty of people are. You can re-use this over and over.
The grips are straight on the outside (the bit that shows in your hair) and a crinkled bit on the inside (the bit that doesn't show) with a rounded bobble on the end which saves your head from being scratched firstly, but also saves your fingers when putting them in or taking them out and if you leave them lying on your floor they won't hurt as much when you stand on them which is a result!

A good buy if you look after them properly.