One of the things that's kind of fun about manga, especially if you read it long enough, is being able to watch the progression of an artist's skills over time. You really don't get to see this with western comic book artists, because the staff changes so many times over the course of a specific title, and the artists usually have little control over the story. But, with manga, no matter how the long the series runs, if you wait long enough, it will end and you'll get to see the principle artist go to the next title, with new stories and possibly new character designs.
When I first came to Japan, Yaiba was still running. It ended soon after, and Conan started up a year later (I followed Conan off and on when I traveled to San Jose on business and could stop at the import bookstore there to pick up Shonen Sunday). Over all, Aoyama Gosho has improved a lot in both his story telling and artistic skills. It's easy to see just by comparing the covers of Kaito and Conan.
Magic Kaitou (by Aoyama Gosho, 1988-2007), Grade: C
This is very clearly Gosho's early work. The character designs foretell that of Conan, but they're crude, childish and inconsistent. The line art is clean and simple and the backgrounds are detailed, but the overall manga is a mess. The story revolves around Kaito Kuroba, an amateur stage magician and school student, who discovers that his late father had a secret identity - the magician thief Kaito Kid. Kaito assumes Kid's identity 8 years after his last appearance, and proceeds to steal the world's greatest treasures in order to track down his father's killer.
Magic Kaito desperately wants to be another Arsene Lupin or Cat's Eye, but Gosho's story telling skills are lacking here. The stories are usually short one-chapter stand-alone gags that don't build on each other. The plots run all over the place, from Kid trying to steal a jewel from a corrupt store owner, to fighting a battle of love against a devil-summoning witch, to being replaced by a robot that thinks it's a human. Worse, the "stage magic" used in the series would never stand up in real life; it's just a little too unrealistic (and mostly rip-offs of Lupin III; unfortunately Gosho is no Monkey Punch). The only reason for reading Magic Kaitou is that Kid makes cameo appearances in Yaiba, and is an occasional menace to Conan, and it helps to know where Kid originated from (although there is one full chapter in "Magic Kaito" where Kid goes to Yaiba's house and tries to steal Yaiba's sword, and with chapter 24, Conan shows up as his original teenage self to face off against Kid). The series was more-or-less suspended so that Gosho could move on to other things.
Summary: A weak series that points out most of Gosho's flaws as a young manga-ka. Recommended only to fans of Conan.
Yaiba, (by Aoyama Gosho, 1988-1993), Grade: B+
Yaiba is another rip-off title. Where Magic Kaitou used Arsene Lupin (and by extension Lupin III) as a starting point, Yaiba used Dragon Ball. Yaiba is another gag series, but this one revolves around a young would-be samurai that starts out growing up in the jungle before being brought to Tokyo; as part of the task of growing stronger he chases after a series of magic balls ("hello, this remind anyone of anything?)". The artwork is still clean and detailed, and the character designs don't overlap with Magic Kaito. Yaiba is a kid's fantasy adventure series that involves lots of silly talking animals, dumb villains and the total and complete destruction of every building in Japan. Fortunately, Yaiba doesn't take itself seriously, so it's easy to accept it as the cotton candy that it is. Unfortunately, it is pure fluff, so it only gets a B+ rating.
Summary: A mindless romp across Japan, as the samurai-in-training Yaiba prepares to defeat his arch-rival, demon-sword-bearing Onimaru. Recommended if you need a Dragon Ball fix.
Meitantei Conan, (by Aoyama Gosho, 1988-1993), Grade: B
Ok, I probably don't have to tell anyone about this title; it's gotten pretty over-exposed in the U.S. under the name "Case Closed". What I will say is that I don't really like it. The artwork is clean and detailed as always, but the character designs were recycled from Kaito and Yaiba (after being significantly cleaned up). What I dislike so much is that the basic premise is so silly - a high school detective gets shrunk, and outfitted by a mad professor with a dart gun/voice changer, super-powered shoes, and a soccer ball-ejecting belt. It's like a toy company hired a manga artist to develop a 700+episode running ad for their kids products.
As always, Gosho needed a jumping-off point to start from, and Conan's was a mix of Sherlock Holmes, Kindaichi Case Files, and other older "Hardy Boys"-style Japanese stories. And, in a way, this is another source of annoyance for me. In all these kinds of stories, the endings tend to be so consistently preachy - no matter how justified a killing is, it's always the perpetrator of the current case that is in the wrong because the "boy genius" was able to catch them. The moral seems to be - "hey, you're a victim, shut up and accept it. Society requires victims, and you're it. I was the one brilliant enough to catch you, so now you must suffer *even more*!!! Bwah hahaha!!!" Unless of course if the victim is rich, in which case Conan arrives just in time to save him. Anyway... No, I don't like these kinds of stories.
Summary: Precocious boy detective outsmarts villians and out-thinks police. Recommended only if you're bored. I read it only to find out how many cliches Gosho will recycle this time. Yes, Gosho has improved with time and practice, but there are still a lot of artists out there that are better than him.
(Spoiler: In Magic Kaito chapter 19, it's revealed that the original Kid (Kaito's father) was killed after trying to steal a large gem. There's a secret organization that is also after the world's largest gems since one of them contains the ability to grant immortality, but the organization doesn't know exactly which gem it is. They kill off anyone in competition with them for the gems, giving Kaito a reason to keep harrassing them. This is the same organization that shrunk Conan into small child form.)
Incidentally, if you do like Aoyama's manga, then you'll want to visit his museum if you ever come to Japan. It's on the west side of the country, quite a ways away from Tokyo, on the other side of Kyoto, though.
Overall Summary: We can see Gosho evolve his artistic skills and character designs, starting with Magic Kaitou all the way up to Conan. For this reason alone, it's fun to read Gosho's work and watch his evolution as a manga artist.