In my experience "re-flowable PDF's" have been a sad joke. Even the BBeB re-flow capability could use some tweaks.
BBeB can be used as a page layout, in a manner similar to PDF. But it also supports auto-layout, you can feed it large blocks of text and it will layout the "pages" itself. Of course on the Librie what humans thought was a "page" and what the Librie thought was a "page" were different. Basically all the text in that large block was a "page" to the Librie. So your book could have 33 "pages" and actually be over 1000 page flips to read.
These large blocks of text limit the number of "short pages" you get when the Librie stops formatting at the end of one block and moves to the next "page". Large blocks also allow the LZW compression to produce a smaller ebook file.
If you use BBeB to do page layout, in a manner similar to PDF, you get one "page" per "page" but you lose some of the re-flow when you press the magnify button. You also get a bloated ebook file. When magnified you get short little pages stuck in between full pages, a bit annoying to read.
This was one of the questions I submitted for Sony to answer: any improvements to the BBeB format coming in the Sony Reader to address these issues on the Librie? No answer was given. If they had something to crow about I suspect they would have been crowing.
Another issue is a real italic font. In BBeB text blocks there is a pair of directives you can drop into the stream of text that pop you in and out of "italic mode". But the Librie used "fake italic", it just sheared the roman font. Looks like italic to a layman but is an abomination to a typographer.
Also, re-flowable text blocks support super/sub script, italic on/off and a horizontal escapement. So far that's all anyone has been able to tease out. There isn't even bold support.
So long as you like your text in one font, with quasi-italic, and super/sub scripts you get excellent re-flow via the magnify button. Otherwise things start getting strange as it tries to shift around page layout directives with its tiny CPU that is trying to get back to sleep mode as rapidly as it can to try and get 7500 page turns per charge.