I have almost 100 O.P.I Nail polishes - I love the formula for the regular colours (no glitter etc). The range is exciting and impressive, when a new collection comes out the marketing and packaging is well thought out.
The glitters need more layers/build up especially the chunkier versions, also when removing glitters treat it like gel polish and soak your nails for 10 minutes or so and it will come off a lot easier.
My nails are naturally very weak and brittle, so the polish doesn't last long for me, but I love experimenting with new colours/combinations every few days.
My favourite colour: 'Mod about you'.
I have quite a few OPI polishes now and i love every single one. They have a range of gorgeous and unique colours too which is awesome. They never chip so easily, like other polishes tend to do. They aren't opaque either, so you only ever need to apply one or two coats and you're good to go! A little pricey compared to other brands but definitely worth the money!