Busou Renkin can only be described as a remarkably average anime that attempts to step outside the box at parts, but essentially remains a slave to the shounen demographic that it desperately wants to please. I read somewhere that this was an anime that only loyal Shounen Jump fans should follow and whilst I do think this is a fair thing to say the anime deserves a bit of extra credit.
The story is painfully generic, boy discovers superpower, boy wants to use the superpower to become stronger and to protect the ones close to him, boy gets more than he bargained for and has to fight his way out of every situation. Whilst the developments mid-way through with Victor etc were, for me, an unexpected and refreshing twist to the plot, the subsequent episodes played out a lot of predictable story elements that were obvious from the beginning since so much of this anime is borrowed from other series - there's no denying the identical nature of the Alchemist Warrior's Busou Renkin and a Shinigami's Zanpakutou.
Busou Renkin is particularly full of frills to distract you from the predictable plot. Along with the change of pace in the storyline halfway through, the character designs are another frill. Are we really supposed to take these villains seriously? Inbetween Papillon's crotch shots and Moon Face's "moooon" catchphrase, everything feels rather confused and questionable. The homunculi, however, were a welcome change from the typical demon-type enemies that grace the shounen scene yet they remained largely unexplored throughout. That said, at the same time one may find these unique characters a redeemable factor within the series.
Unfortunately, not much can be said for the rest of the character line-up; Tokiko seems to be the only character with any depth yet her relationship with Bravo did not seem as developed as it should have been given the flashback that is revealed of the characters' first encounter. Kazuki remains quite one-dimensional throughout; his development is all in the manner of accepting his power and becoming stronger, the attempt to make his character broader is by throwing in the romance between him and Tokiko. And hence the generic but comedic beach fun bikini episode. Victor's character was quite interesting but left the anime in a philosophical quandary toward the end; are we not to assume that Victor's godlike state would leave him without the emotional capacity to bother partaking in any fighting - yet he insists on destroying all Alchemist Warriors.
The music felt out of place and overdramatic, and the OP and ED were so forgettable I don't even recall them. The English dub gave Tokiko and Kazuki voices that sounded too mature for their age, and the Japanese cast was purely okay, but failed to bring true life into the characters. Angel Gozen was voiced particularly well, and I was pleased to encounter him/her in multiple episodes alongside our protagonists.
Obviously, this is written from the perspective of someone who can watch shounen and enjoy it, but ultimately wants a little bit more from anime. Even as a shounen, though, there are better ones out there than Busou Renkin.
Overall, worth watching through to end. Whether it be for the curiosity behind the manifestations of the various Busou Renkin or to watch that oh-so predictable ending just to feel a little bit smart. However, don't choose this one over a series that you may be dying to watch and don't expect to become very attached to the characters, or engaged within the story... especially with the 10 minute fight scenes. Shounen fans, look no further, you will love it. People new to shounen may want to check out some of the recommendations before watching.