Having watched this topic for quite some time and avoiding migrating to Mavericks because of a fear of the FW drive not spinning down, I thought it was worth recounting my experience, particularly as it seems counter to that of most.
I have a 2008 iMac and a G-Tech G-Drive fifth generation (FW800 and USB3), and have just done a clean install of 10.9.3, and then migrated my data using the Migration Utility. The G-Drive is used for Time Machine and for Carbon Copy Cloner image copies, so is not used in "normal" operation. I do have the Energy Saver preference to sleep drives when possible.
What is my experience with using the G-Drive attached via FW800?
When using the system, the drive does spin down after some sort of timeout, perhaps of the order of 5 or 10 minutes. As soon as I go to Finder, it spins up.
When I shutdown, the drive spins down immediately.
When I sleep the system, the drive does spin down, but only after a similar timeout of 5 or 10 minutes. This is the only area of difference with my previous use of Mountain Lion, which spun down immediately in these circumstances.
So, I am pretty happy with this. I wonder if my use of a clean install helped, as there are G-Drive users who complain of the problem but G-Tech reckon they have never experienced the problem?