Sometimes it feels like yesterday that "Eien" came out but the song is already seven years old.
What makes "Eien" so good for me is that it's a mid-tempo song with a melancholy sound, but with a beat fast enough to keep me awake. If mid-tempo songs are up your alley, this is easily one of BoA's best mid-tempo songs.
The thing that saddens me about "Eien" is that this was really the last time BoA was relevant in Japan (spring of 2009). This essentially coincided with my waning interest in J-pop. BoA still has released great songs since then, but avex has largely given up on her with the landscape changing in the scene.