The version of Thematic that is currently in the wordpress repo is 0.9.7.8.
The next version will be 0.9.8, but before it is released that link provides a development release for download. One can say it is in a between state, not 0.9.7.8 but not yet fully 0.9.8. The development version is named from the SVN revision, i.e. rev756.
It is true that sometimes the latest SVN can be in a broken state, and that's why Chris put that link up so people can download a version that is stable enough for use. But a lot of development has happened recently in the SVN and he has not yet updated the link. The version number in the SVN files has been bumped to 0.9.8 and it's on the verge of getting submitted to the wordpress repo so I would say it is stable enough to use. The latest revision as of writing is 842, and if you know how to get the latest version from SVN, I think you can do that without too much worry. There are some things that you can take advantage of there. I think for example the theme options that have been updated to use the Settings API is not in rev756, but in rev842.
It has been tested a bit, but if you do happen to find any bugs or unexpected behaviour, please report it at the googlecode site so it can be fixed as soon as possible.
In short, from older to newer:
0.9.7.8 - the wordpress.org version
rev756 - the development release currently available at http://developing.thematic4you.com/thematic-development-release/
rev842 - the latest and greatest soon-to-be-0.9.8 in the SVN
EDIT - hah Helga beat me to it :)