When compared side-by-side with the manga of the same name, the Pet Shop of Horrors anime suddenly looks shoddy and duct-taped together. However, as a stand-alone anime it is very unique.
Each episode can be watched without having any background knowledge of the series, as they are all their own story, similar to anime such as pokemon were it is easy to jump in at any point in the series. However, this is also a drawback, as were there should be a pause to explain something further or in more detail, the show chugs along over it, crossing its fingers and hoping that the audience will understand.
As dark anime go, Pet Shop of Horror doesn't necessarily have a unique concept, but the overall story is unique. It's easy to become lost in the episodes, which provide a unique "this-could-really-happen" sense that few others manage to achieve.
The animation leaves something to be desired, however, with even the Japanese, English-subbed version looking like a terrible American dub. The background music is interesting, but the anime also keeps things quiet were there should be ambient noise such as birds, cars, even the wind.
Character development is limited, but as the series is so short it's not so much that they didn't bother, its that there simply isn't enough time to get to know the characters and watch them grow.
Overall, if you watch anime for the music or animation then this isn't for you. Fans of the manga, be prepared as these episodes are based almost word-for-word on stories from the first three volumes.