The first thing to mention, is that Busou Renkin uses terms like "alchemy" and "homunculus". I want to make this clear, Busou Renkin is not a full metal alchemist rip-off. (because that's what I thought it was when I read the plot summary... boy was I wrong.)
Busou Renkin's story is fairly simple "us" vs "them" goodies vs baddies. But it's the way it's executed that makes the story great. Although there are not many actual plot twists as such (there are a few good ones, though) the plot manages to keep the viewer guessing. What's going to happen? Who is the main villain? How will they defeat them? All these questions are asked, and depending on which episode your at, they have totally different answers. You might be able to guess where the series is going, but it'll jump in leaps and bounds. What might take another series 10 episodes to do, Busou Renkin does in 3. This is a good thing, because it means there is little filler in Busou Renkin, all the battles and every episode serves to progress the plot/characters. It's a well-lengthed series, that really ramps up at around episode 7.
One thing to say on this. It's "magnificent". The battles are great, the CGI is well used, but more prominently are the character costumes. This anime has some of (if not the most) memorable characters ever... (although, those that have seen the series, sometimes wish they were not so memorable...)
The intro theme in particular is one of the most catchy, hot-blooded and awesome pieces of music in anime. The in-series sound effects are good, and likewise the voice acting. I couldn't fault the sound at all.
This is where the series shines. The characters are ... well. Words simply can't describe them without plunging into spoilers. Let's just say, the characters make the series. The are unique (which is a hard thing to do in modern anime) and there are enough of them for everyone to find at least 1 character to love or hate.
Busou Renkin is a thoroughly enjoyable series. I could liken it to the comedic-yet-serious nature of Trigun or Kenshin, and I say it's one of the best.
It's not for everyone, if you want a serious, dramatic anime, your out of luck here. But if you want a good, enjoyable, action packed, but believable anime, Busou Renkin is for you. It really must be watched, and then you can come to your own conclusions. But probably the biggest stand-out feature is how unique it is.