I've tested the four built-in apps with my ePub edition of "Pride and Prejudice" from the MR library. The results are:
AIReader:
Centred text is not centred. Ignores hard page breaks. Ignores font size changes. Ignores italics. Very bad.
FBReader:
Respects hard page breaks, centred text, font changes, etc, but (a killer) ignores first-line text indents
.
OnyxReader:
The best of the bunch. Respects all formatting, but pictures blow up with font size and overflow the page.
Cool Reader:
Centred text is not centred. Italics are ignored. Page breaks are shown properly.
The only half-way decent reader among them is OnyxReader, to my mind.