The ones I've tried have been great. Highly pigmented, smooth formula. I think they did gel versions of their popular colors, and part of their collections as they come, but not all. It seems like they have a few near dupes so make sure to do research, which is easier thanks to lots of swatches of their RNP.
OPI is usually $4-5 more than Gelish, sometimes a lot more. I find them cheaper on EBay, but there's more risk there with private sellers.
The biggest issue I had was the lamp. I originally started with a UV lamp. I did a couple of OPI colors without any really obvious issues. More color seemed to come off with the top coat than usual. The polish didn't chip sooner, but was kind of dentable. I even had a swatch that turned out horrible (wrinkly, chunky, and patchy) and just chalked it up to a bad bottle. Then I heard OPI polishes can be tricky when used without the OPI lamp. I'd been wanting to get an LED lamp anyways, and most brands aren't as picky, so I got the OPI lamp. It seriously made all the difference. I don't know if my UV bulbs were old, it was just using bthe LED bulbs, or specific OPI LED magic. So I guess, if you don't have the OPI lamp, beware and start small.
I attached a picture of the polish that swatched horribly with my UV lamp (left) and then reswatched with the OPI lamp (right). Both were three coats.