Rental Magica is a bit of an odd anime. It's mainly a series of one shot episodes relating the exploits of Iba Itsuki, the 2nd president of Astral, and his various employees as they combat evil spirits, demons, rogue mages, magical pollution, etc.
What makes the show a bit odd is the fact that it initially can't decide if it wants to be a combat anime, or a highschool romance, a purely supernatural thriller, or a comedy. It literally has a bit of all those genres. Because of this, many people may be put off watching the show after a few episodes. Bear with it though, as it does get progressively better.
The story in itself, as explained already, is mainly a series of one shot episodes. There is, however, an underlying story which is hinted at throughout most of the series, and becomes clearer towards the end (which is why I said bear with it).
The art and animation in the show is generally quite good. The characters and backgrounds are generally very well done, and the majority of spirits/demons/etc, are very well depicted. The combat scenes are very well animated and should satisfy most people.
The sound is also well done. The opening theme is a catchy J-rock song which has become a favourite of mine (both in japanese and english). The sound effects and atmospheric music are generally good throughout the series.
The characters are reasonably well realised. The characters of Itsuki, Honami and Adelicia are generally quite good, and an effort has been made to focus on specific characters in several episodes scattered throughout the series. This helps to round out the characters afair amount, but otherwise there is no real effort to develop the characters more - but then again, this type of anime doesn't really need character development.
I'm a bit of a fan of episodic animes (Mushi-shi, Aria, Mokke, Ghost Hunt, to name a few), so watching Rental Magica was an enjoyable experience for me as I didn't expect any distinct plot to appear. I also liked the fact that this show highlights one thing that is commonly left out of other "magic" based shows - the rules.
I'll explain. Astral is an organisation made up of practitioners of various forms of magic (Celtic Witchcraft, Shinto, Onmyoudou, etc - they even have a ghost working for them). Astral isn't the only magical organisation though, as there are numerous others (Goetia - the descendents of Solomon, is the most notable). All of these organisations are governed by a mysterious organisation simply called "The Association", who enforce the rules regarding the use of magic. Certain acts are considered taboo, and the show covers some of these very well.
The other side of the "rules" coin are the rules governing a particular form of magic. The show very clearly highlights the differences in the various forms of magic used, and also covers some of the dangers of misuse or breaking one of the taboos. This is what really sets the show apart, as it is one of the few shows that covers the technical side of magic, and it covers it quite well. The creators of the show went to a lot of effort researching the various forms of magic in an effort to make the various rituals as accurate as they could, and for me at least, the effort paid off.
The only thing I didn't like about the show was the character of Itsuki Iba, as I found him to be a bit too shy/reticent/cowardly/etc. This is supposedly balanced by his alter ego (when he removes his eyepatch), but initially the difference didn't sit well with me as I kept wanting to punch him in the face (I should point out that he does improve later in the series though).
This isn't a straightforward combat anime. Nor is it a truly supernatural thriller. It most definitely is not a horror, although some of the uses of magic in the show can be called horrific. It's a complicated mixture of several genres which may, or may not, sit well with most people, depending on your preference.
If you like coherent, sensible storylines with good plot and character development then you may not enjoy this (doesn't hurt to give it a try though). If you're after thrills, action, chases, etc, then you may not enjoy this either.
If you're after an enjoyable anime that isn't too serious, and is sometimes a bit tongue-in-cheek with it's humour, then you may actually like this show.
I can think of worse ways to spend half an hour.