In my opinion, Phantom Requiem will may be figure in the top10 anime of the year 2009.
Phantom Requiem is firstly an anime where you need to be attentive, in fact, mafia members in that anime often discuss about plans,strategies which are quite difficult to understand.However, the most fascinating side of that anime resides in the relationship between Reiji/zwei and Elen/Ein.I would say that the main theme of this anime is FREEDOM.
Phantom Requiem is an anime for those who mostly love drama since the story of Elen and Reiji is very touching.Therefore, if you are searching for impressive scenes of actions and suspense, I suggest that you don't expect more from this anime.
Strength of this anime:
1) Drama is very good- following how Reiji becomes more and more close to Ein is the main strength of this anime.In fact,Reiji and Ein share the same suffering and this creates a strong bond between them.
2) Art is extremely good- main characters and even side characters are well drawn.
3) The music is good- Opening(Karma) is damn good and background music(which are mainly gothic type) are perfectly adapted to a given situation
4) Many many many moral values which i'm sure will teach you many things.E.g Once you killed someone, killing again will become easier
1) You will probably hate 85% characters of this series since they are always looking for power.This is a real weakness since those characters will have consequent screen time.
2) Too many discussions among characters which is harsh to understand but hopefully, you can understand what is going on even he you didn't understand what was being said.( It's like politics XD)
3) Probably, this anime in my opinion had a very slow start but things will become more and more interesting around ep12 ( however, it was interesting to follow ep1-11)
4) Lack of suspense at the end of most episodes which I think is quite unfortunate.
Basically,this anime is great and you should definitely gave it a try.