Indigo 6 v6.0.18 (and higher) is functioning properly on Yosemite. We still do not recommend using Yosemite for critical applications/services, including running Indigo. We are continuing to QA and resolve Yosemite compatibility issues as we find them.
There does seem to be some outstanding issues with the Silicon Lab's VCP driver, which is needed to support the Aeotec Z-Stick. We've not been able to replicate the issues, but we have several users reporting problems. If you are using Z-Wave and the Z-Stick, we highly recommend waiting to upgrade until more information is known about this issue. Unfortunately, there's nothing we can do directly since it's a problem with the driver itself and not an Indigo issue.
Unlike the latest version of Indigo 6, Indigo 5 is not Yosemite compatible. Reflectors will not work and there may be other errors / issues. The changes to Indigo required for Yosemite compatibility were significant enough that we won't be able to make Indigo 5 Yosemite compatible, so users that upgrade to Yosemite will need to upgrade to Indigo 6 for full functionality to continue to work.