Hmmm... I don't really understand why the majority seems to recommend (color) tablets!? Mangas != comics! As maledyris already said, mangas are black & white ONLY. (except for a few pages) Therefore there is no need to buy a color tablet.
I would go with an E-Ink Pearl device - namely Sony or Amazon. Here you can find some impressions of how mangas look on a kindle 3:
click
I would go with the kindle 3, or prs-950.
regards.
bearmann
PS: First post on this forum. So a nice 'hello' to everybody out there.