Insomnium plays a melodic death with tendencies of the Swedish metal (that is production values (slightly), somewhat similar to early In Flames, and a bit Dismember)
The main quality of the music is its wonderful songwriting. The music is wonderfully and carefully crafted. The melodies are simply brilliant. From melodic death metal perspective this is an important to have interesting melodies. And they are consistent; every song has an interesting hook, acoustic guitars and keyboards to enhance already dense sound. The other instruments are very nicely done too. Vocals are brutal, yet clear, and very emotional. Plus, the lyrics are thoughtful and poetic. Rhythm section tends to be not very complicated but not uninteresting enough. Check out the double base work in the 'Black Waters', or very powerful drumming in the 'Dying Chant'. It seems that songwriting is done by the guitarists, and that helps to understand their songwriting techniques. As far as not being original (oh they copied from In Flames and suchlike), there are some similar work (the cute little guitar lead in ‘Journey Uknown’ sound quite similar to In Flames earlier material)
Yet in general, Insomnium are quite original. The combination of heavy production, thick guitar wall of sound, melanholic melodies,interesting changes, plus the atmosphere. The song on which they made a video characterizes their unique approach. And there are no fillers. The last title track is so amazing, such as beautiful song with such an emotional impact (the last 4 minutes of it are orgasmic – million stars out of ten)
Last bits:
I give it 95/100 there are some repetitive moments in few songs, (Journey Uknown lead comes back few times, it is a nice little thing, but gets repeated a bit too much) but it is pretty damn close to perfection. Additional respect for the artist to avoid trends. They have no metalcore influences, and no modern pop sampling and suchlike, which means they are not really commercial project, and writev songs from their hearts, plus it is nice to avoid the fucking trends, and stick to the roots which, from a listener perspective, is very enjoable. On of the amazing original artists in the overcrowded genre.