Most of the episodes vary in quality, some are amazing while some aren't as powerful though not necessarily bad just not as good (example:the episode about the rainbow). The effects and the purpose of the Mushi were pretty diverse which made it feel almost like a different story each episode. The show can be sometimes quite tragic, not every episode end in a good note, though I wouldn't say it's dark, it's more like the show isn't afraid to shy away from the typical happy ending.
Animation:
The character design could have used a bit more diversity, since every literally looked the same, I understand that it's pretty much the same case with most anime but everyone had practically the same haircut and facial features that some people would think hey didn't I see you last episode?, at least Ginko actually looked different, though the plain looking characters are easily forgivable because the overall quality of the animation is top notch, like the fantastic looking environment, a lot of highly detailed scenery that helps giving the show an a feeling of epicness.
Sound:
I watched the dubbed version which was actually quite decent, there was virtually no awkward dialogue and it the voice actors didn't sound robotic, there was some instances where the some voice actors were used repeatedly for different characters but it's not really such a big issue since most games and anime dubs abuse this even more and to be honest I'd rather have that than shit voice acting that most dubbed versions of anime have.
The OST usually consist of subtle (and effective) guitar and piano scores, the OP was good I suppose it's a little bit too happy for the show though.
Character:
Ginko is a good main character he's not annoying but I felt that they could have handled his character a bit better since he doesn't really have that much of a personality, at least the show had one or two episodes dedicated to his back story which prevented Ginko from being a boring character. The show does it's best trying to make you care for the side characters during each episode, while they do a good job making you care even with the fact that the characters look practically the same.
Overall:
Well I have to say episodic shows are possibly the home of great anime; Aria, Kino's Journey and Ghost in the Shell: Stand Alone Complex (too an extent) are all episodic shows and they are definitely anime of the highest quality, these kind of shows manage to make each episode different and interesting, even as most of the time they have a recurring theme each episode, it never feels repetitive.
Mushi-shi is definitely one of the best anime out there, it might have some faults but it has a lot more aspects that it does pretty well. it's a show that's absolutely impossible to hate, and is definitely something every anime fan needs to see.
