In my experience they got pretty close to Pandora's recommendations already, which were excellent.
last.fm for me just recommended the next big artist in the same category. Often their music styles were still obviously different and the recommendation rather put me off. People that are hooked by the complexity, details and perfectionism in "Nightfall in Middle-Earth" won't necessarily like Manowar.
I recently got one Spotify recommendation that lead me into listening through a band's full catalogue and getting tickets for their show two weeks later (Insomnium, btw). It also dug out a song that I liked in primary school but completely forgot about. They discovered that I like cheezy metal covers of 80s pop songs and add one by some obscure band to my list every now and then. I'd say my experience was often pretty accurate.
Thanks, that is one of my main gripes with most recommendation engines, and since you like metal, the examples really resonated with me. For me, it was Wardruna - an ambienty, folky acoustic group. Listeners often also like the "typical" viking metal bands, but damn, if I want something like Wardruna, I do not want death metal singing about the same topics, I want, say, Forndom. The same often happened in electronic music as well: if I listened to 5 relatively unknown psytrance songs I liked because they have a nice balance of melodies layered with very distinct synth sounds, then no, I don't want to liten to David Guetta next, because that is also trance-like.
In other words: yes, last.fm's recommendation was much too much based on "customers also liked", which helps in a lot of cases, but so often it horribly fails ("Customers who bought The Martian on Blu-Ray also bought this asthma medication because chance happens. Wanna try it out?").
Last.fm's customized radio didn't work well for me - it'd play the same limited selection of tracks every time, often one song from an album and never play any of the other ones on it.
I disagree, Last.fm's recommendations have always felt lukewarm to me. Discover Weekly, on the other hand, has introduced me to TONS of new music that I still regularly listen to.
