Yes, there is. From any song in your library, a simple right-click (in iTunes) or a tap on the "..." next to a song will bring up the option to "Start Station" based on that song. Instantly, you will have created a "station" that plays music similar to the song you picked, and appears alongside the other "radio" stations in the "radio" tab of Music/iTunes.
Once a song is playing from the station, expand the mini-player to see a "star" icon -- this gives you the option of saying "Play more like this" or "Play less like this," which is less customizable than it was under the previous version. At some point -- but not presently -- we should gain the ability to delete these custom stations. At present, there is no way to do that I can find. Early days.
I also see no way to play these offline, but that is a condition in which I rarely find myself, so perhaps if you are offline and press play it will search your library for songs that fit -- or perhaps not. In the previous version, you could choose an artist and then add additional similar artists yourself and such -- that may return, but its not present now. Again, early days.
