Tikiwiki 1.9 is currently marked as stable / testing, However, it is fairly new.
There are plenty of new features and redesigned sections in Tikiwiki 1.9, ReleaseProcess19 has the full details
Upgrade details are on UpgradeTo19. CVS checkouts are likely to be more stable at the moment.
See Install19onIIS for details on getting 1.9 installed on IIS servers