Yes, those look like changelogs. But I've spent entirely more time than I like to consider trying the different ROMs, and I agree with ev1l. What would really be helpful is not a changelog from one version of a ROM to the next, but a list of the differences (operational, software support, hardware support) between the original Sharp ROM and the other ROMs that are available.
ev1l: I'm a newbie, too, but so far it looks like:
The current Cacko ROM is essentially the same as the original Sharp ROM, with some small but useful improvements. Mainly, it is entirely in English, is very stable, and has a Console application built in. Cacko seems to be compatible with the largest set of third party software.
pdaXrom seems fast, but not too stable. Screen rotation is partly supported, but not enough to be useful in portrait mode, and it does not come with any handwriting recognition. A fair amount of software is available for it, but some of the applications have not yet been adapted too well for the small screen and other limits of the hardware.
OpenEmbedded/Opie looks great, but the current version is not compatibile with very many third party applications, and its also somewhat unstable. Openembedded sounds like a good idea, but it may be too ambitious for the size of the team they have working on it.
KDEPim has not been fully adapted to the Z yet, but already it is a very useful PIM, particularly in combination with the KDE Desktop versions. I have it running on Cacko, but was not able to get it to work under pdaXrom or Opie.