I picked this up because of VivisQueen's review and I was pleasantly surprised.
First thing that caught my attention was the awesome OP song by Galneryus (ED too), Yama-b's vocals are just epic. Other songs of theirs are worth checking out too.
Apart from the 6th ep the plot was interesting enough and I especially liked that apparent flow of time between episodes. It might have been developed further, answering some questions about the past and having better motivation for the cast, but it has definitely done a good job. If you like mystery plots, you should at least try it.
Characters were like a breath of a fresh air, but that might just be because I have recently finished Gantz. Anyway they were likeable enough and well written (and by that I guess I mean Rin :)).
I am really glad that it is not overdone on the explicit content department. It really is hard to balance these things, the obvious and easy (and usual :() way is just cheap and not even worth watching. I dare say Mnemosyne has done it quite right.
I would like it to have a different ending, but all in all it was an enjoyable experience.