Death Billiards is GREAT stuff. Deserves a 4/5 on douban hands down. I think that if this OVA was longer (like a 1 hour 30 minutes full-length movie), it could have potentially been ranked in the masterpiece category, for only some achievable conditions were met. Yes, if the themes of the show that are already here were more developed, if the animation & visuals were just as good, if the cool factor was continuously played & timely placed as a surprising variation of the show's flow, if the psychological & very human part was even more emphasized and developed... etc, then it would have deserved this status.
But since Death Billiards is only a 25-minutes anime (just as long as an average series episode), it's hard to see if the show deserves so much praise: it's much easier to do a goddamn job when there is not enough time for chara-development (and therefore no possible evaluation on this criteria), but just enough original & thought-provoking ideas and high production values for impressing the viewers. Whether there is proper development for the ideas and the mysterious world in which the events are taking place, the duration does not allow us to answer these questions (for the OVA of course, the series that will come later does, and partly fails).

Essentially, the concept of the OVA is similar to a social / psychological behavioral experiment, in which the subjects are not real humans but characters in a show, and most of the experiment being the "fruit" of the creators' imagination mind. Instead of grabbing your thickest glasses, and getting ready for some lengthy & complicated research paper, you can just kick back, relax & enjoy your anime dose. Sounds cool, right?
Death Billiards' events are very similar to the experiences in which you observe how people react & behave whether they are prisoners or prison guards; or the experiences made to check how people behave under orders from authority (including whether they can literally kill someone begging for their own life). In this anime, we see how fictional characters (mirroring real people) think & behave on death's aftermath, on the day of the Judgement. Having an access to their POV, something that science can't do yet lol, is definitely interesting and enables a full understanding of the characters motives and logic. Their deep nature, true personality and core values are enhanced & revealed in this artificial situation that the show creates for them, and even more so since the characters might think that their actions can influence their judgement: it's either heaven or hell, or simply death or survival (the characters do not know that they have already died until their memories start rising to the surface after a while).
So it's an interesting situation that would definitely interest any psychologist if the experience was possible. Basically, the Arbitors (the masters of the aftermath, judging dead people) force people to play a game (here: billiards) before mentioning a few rules, but leaving a lot of room for interpretation and creative thinking: nothing for example forbids the characters to beat each other to death, as far as they play the game later (even if one of the players is knocked down).
Also the choice of the game looks as if it was chosen randomly, when it seriously advantages one of the parties. Heck, even fighting with a stick is clearly in the same character's favor... So what does that mean? Are the judgements fair and square? Or are they mirroring life itself in its injustice? Is it possible to turn bad odds in one's favor? Is the result of the game that relevant after all, or are there other criterias that are taken into account to judge one's soul? To what extent the arbitors are telling the truth as to the role they are playing? A lot of questions arise during the visioning experience, including some that cannot be answered yet. An ambiance full of mystery surrounds everything in this show: the main characters' who seem like dirty manipulators, the huge, dark & luxurious place from which one cannot escape, dead people's fate, etc...
The characters behave in a very natural & realistic way, the cast being very convincing. The production values are very high, both in terms of sound (effects & music) and visuals (chara-design, settings, animation).

All of the above makes Death Billiards a great one-shot experience to be fully enjoyed around a good glass of wine or cocktail.