Mnemosyne has been noticeably losing viewers as the numerous fanservice dedicated scenes become more evident, but since fanservice has become a staple in recent anime mostly due to commercial reasons, it is pointless to dislike a series based heavily on the amount and harshness of those.
One of the strongest points of the series is the interesting portrayal of immortal beings and the evolution of society and technology throughout the multiple decade time-skips between each episode, the aging of mortal characters and their long but passing relationship with the main characters. Compared to most other anime that do have long-living characters in their host, Mnemosyne presents a much more realistic and accurate view on the aspect, arguably comparable to that of the western classical RPG Vampire: The Masquerade.
The antagonists, while not presenting many, if any, noticeable original characteristics not seen in other works, manage to stay mysterious to the end, with very little of their motives, personalities and goals being apparent to the viewer, as well as their natures and origins, resulting in interesting but often small revelations occurring in every episode, successfully enticing the mind of the public.
The protagonists, as immortals, largely resist changes in personality and habits, and, as previously mentioned, maintain interesting relationships with the supporting cast, aware that those are fragile just as the lives of innumerous beings they have watched fade away throughout the ages, but also not treating them as disposable or of low value. Both are shown as having a substantial amount of knowledge, and as preserving the memories of those who were important to them. The main character's fighting ability is not as great as one would expect, but is overally consistent, without noticeable bursts of martial geniousness or inexplicable failures on sure-win situations.
In the end, Mnemosyne can be viewed as a like-it-or-hate-it series, as it's merits are notable, it also requires a lot from the viewer to be appreciated, specially on the western side of the globe, due to it's heavy graphics and plot. It might also become a vanguard work, opening more space for future similar anime to bloom.
