The first stop on our tour is eBookJapan. The good: massive selection, including major publishers' crown jewels. Not so good: volumes generally cost the same as physical manga, with maybe a 20% discount for older releases (like, really old). The bad: the Android app has terrible ratings. Personally I haven't had that much trouble with it, but it's rather bare-bones in functionality compared to competitors' apps.
This is going to be a lazy review because I'm just going to complain. When I try to make a purchase on the tablet, the website just gives up. When I try to purchase on PC instead, it tells me that I need to install the PC app. Maybe I'm being hardheaded, but I think this is really stupid because every other eBook store I've tried is perfectly happy to let me pay on the PC and read elsewhere.
Anyway, I'll probably come back to these guys at some point.