I.O.I is a group that's easy to like even though their music is fairly meh. "Dream Girls" and the random Salt-N-Pepa cover "Whatta Man" were both passable pop singles, but hardly worthy of a group that has become of the most popular forces in K-pop this year.
Just like a female version of ZE:A, Rainbow are famous for being flops, yet they're still successful enough in various ways to hang around despite never really hitting it big. Their singles usually make the top twenty, and between the members they've got some CF deals, acting gigs, and variety work--basically just enough to get by in the Korean entertainment industry.
I consider myself a fairly big Rainbow fan, having loved every single they've put out since "A" (with the exception of "Hoi Hoi," obviously). Their latest, "Black Swan," is more left-field than I expected from them, but it was instant love from the first spin.