I am a complete OPI fanatic.
98% of the polishes I own are OPI and I am rarely (if ever!) disappointed.
The shades are beauuutiful with so many options for all skin tones and all tastes. Great range of shades within colours too, eg. huge range of pinks, nudes, grey's, etc.
The brush on the OPI polishes are my favourite of all time, with the wide brushes making application quick, easy, and tidy. You can coat an entire nail in two swipes, three for a thumb, and most colours apply evenly without streaking.
They are on the pricey side, but Farmers and Chemists often do deals and discounts so definitely hold out for those as they pop up frequently and are worth waiting for. Plus you can then get several at a time :)
Also I love how they keep their main range (which is huge) and constantly bring out new collections - OPI never gets boring and constantly brings out new shades and finishes.
O.P.I nail polish are one of my favourite nail polishes to use! I like the creamy formula, how most of them dry or 'set' almost instantly and most of all, the variety of colours they offer! for every single skin tone, occasion, trend and however you're feeling at the time :P Most of the O.P.I colours do look the same in my opinion, like the reds! I can never tell the difference between some of them! I like the more pastel, creamy, nude colours and although nudes are the hardest colours for me to put on without the streak marks O.P.I makes it easy for me with the great combination of the formula and the brush :)
Although they are a bit pricey sitting at $24.90 each you can get them cheaper online or they do quite often go on special :)