I was hesitant to try the new version as other versions of 5 did not respect file associations, once you opened up a document all Pages documents became associated with the new version - once a document was opened it was irretrievable in a prior version. I am happy to say that while the file association behaviour is a bit bizarre it now more-or-less does respects them. If you right-click on a document it says 5.2 as default, but it seems if you specify 4 it will respect that even though the default doesn't change what it says.
When you do open a document, as soon as you try to edit it, 5.2 asks if you want to make a copy. I did this, edited the copy, and then closed out the session. When I clicked on the original I was first met with the 'can't open/unknown developer' dialog, yet when I right clicked (allowing me to open it) it opened nicely with Pages 4 and has respected that decision since. Oddly, if I right click it stills says 5.2 as the default app even though if I double-click it opens in Pages 4!?!
You can add captions to pictures (which is necessary for what I do). Again, the behaviour is different, but there is now an extensive online manual to help. Essentially, pictures and text boxes are both considered objects which can be grouped. To be honest, once I got used to it I almost like this way better. Further, it does seem to 'know' when you are trying to make a caption by sliding elements around. While the prescribed way is better, you can still do it this way. However, the old way of inserting an image into a textbox doesn't work at all. Bullets also seem to work and have choices.
I opened a previously published book of mine to see if it would respect the formatting. It actually did a much better job than prior versions, but some captions worked, some were broken. The Table of Contents was a bit broken, and I have not found how to edit its styles. The help does say it is possible.
Document properties does not list a 6x9 standard book format, but you can create one from page setup under 'file.' Yet, it does not add it to the document list, just saying 'other', which disappears next time you use it.
Also, while writing this, I closed out the documents I was viewing while not exiting Pages 5. As soon as I started typing in Safari Pages shut down... odd...
The over all behaviour is certainly different and less intuitive (which would change through familiarity), but I was able to do most all of the things I could do with Pages 4. There are also several things I found new, such as importing a picture directly from a screen capture if you right-click at an insertion point.
In general, I could live with this, but two-up viewing is still broken, which is very helpful for book publishing. It might be a deal-breaker for me. I'm sure there are more hidden surprises, but at least I can now experiment without fear of breaking my old documents. Of course it would take time to get used to. I can't call it a totally polished product, it still feels a bit beta-ish. This is the version Apple should have released originally with a beta disclaimer.