Last.fm does alright: http://www.last.fm/music/Dark+Moor/+similar
Last.fm puts Galneryus on page 3, and Galneryus is sufficiently different that I wouldn't trust the recommendations that score lower. Last.fm correctly places Stratovarius above this cutoff, but Helloween, Sonata Arctica, and Blind Guardian don't make the cut, and Ayreon is all the way down on page 10 with Van Canto and Turisas. Van Canto and Turisas are great, to be sure, they are just way less similar to Dark Moor than Ayreon is.
Of course, Last.fm is better than nothing: http://ifyoudig.net/mohican-sandbag vs. http://www.last.fm/music/Mohican+Sandbag/+similar
Last.fm just reads tags, so a lot of artists have multiple names. モヒカンサンドバッグ is the second result for being similar to itself! Last.fm also does a pretty awful job by putting ORANGE★JAM and 3L on the first page, with dBu closely following. Much more similar producers and circles like 和泉幸奇, Alstroemeria, or IOSYS are nowhere to be found. Izmizm and Shibayan on page 1 are sensible. So maybe Last.fm isn't all that much better than nothing...
I think this can be good, and I might even pay for it, but ultimately it will be just one of a handful of half-solutions that need to be combined to get quality discovery. Pandora and Google Music's instant playlist do better than I would expect any people-who-like-x-also-like-y similarity system to do. Unfortunately Pandora has like 7 artists in its library and Google Music's instant playlist requires you to already have the music you are discovering, which sort of misses the point.