I purchased my first OPI a few years ago after hearing a friend rave on about the brand and got hooked...
There are so many great colours in OPI's range and i love that they bring out 'collections' with colours that all compliment each other. The OPI bottles are a decent size and the brush has really great coverage, only needing 2 swipes to cover your nail (although i usually tend to do 3 as that is how i was taught to do them!) The OPI formula is not too runny so it gives you a great finish after 2 coats and lasts quite well, however i dont really feel like it has any better staying power than other leading nail polish brands. With the $20-$25 price tag i usually only purchase them now if i really like the colour, as i have found a slightly cheaper brand with the same quality. In saying that i do regularly use the ones i have purchased in the past. All in all definitely a great brand that i would have no hesitation in recommending to others!
I definitely haven't found a brand that beats OPI. They have the best colour range, and I find most colours last really well on my nails. I used to avoid buying it all the time and get cheaper polishes, only to regret my choice instantly! Now I only purchase Revlon fast dry and OPI polishes.
Tips: I find the metallic polishes tend to chip faster than the matte ones.