I have just finished reading The Guin Saga: Book Five: The Marches King (Bk. 5), and I have to say that this series is not uninspiring at all. Then again, I'm a fan of heroic fiction, so my perspective may be considered skewed. I picked up the books on a whim, and, although I can say that the first couple of books are mostly setting scenes as well as introducing characters and the world in general, books 3-5 were more about developing the characters.
Admittedly, Guin not having a memory and being able to do things inexplicably is a big plot device, but even Kurimoto recognized that, and has reminded us in book 5 that Guin is basically a hand of the gods.
I think the twins develop well as the story progresses, and are recognizable as separate beings with their own personalities and ways of thinking. If there was no or little character development, I think this wouldn't be true. Rinda is definitely spunky and too proud, and Remus is too cautious, but watchful.
Also, one may not start out liking Istavan, but by book 5, he too has developed in ways. He's mischievous and cheeky, with a foul mouth, but he's interesting because of all the characters he shows the most flaws.
I should lastly like to say, for the most part, all of the characters in this series are very young. The twins are only 14; Istavan is merely 19; Suni is just a child; and even Amnelis is just 17-18. They are guided and protected by the older and wiser people in this story, but they are STILL children. They are developing even as the story develops. Kurimoto tends to reinforce that they are just young teenagers trying to deal with the turmoil around them, all the while trying to achieve their destiny.
The writing improves as the characters become more rounded, and it's a shame to play this off so simply. The first book may not be ground breaking, but it does lead to a much more interesting world.
I would say give this a try through book three and see if it grabs you then. There must be a reason that this series has captured so many people in so many places for so long. I mean, there are 126 novels that are translated in a few languages. There must be something more than just the first two books can give us.