The adapter works well when it works, providing full FireWire 800 performance. But users of the adapter have encountered a frustrating limitation: Only 7W of bus power is supplied to an attached device. The FireWire standard supports up to 45W, although most computers, including Macs, deliver 10W to 20W. Some bus-powered FireWire devices have an optional DC power port, even if they don't include an AC power adapter. If you can externally power your device, you can bypass the 7W limitation. Otherwise you'll need to explore other solutions.
Even if you can run within the 7W budget or bypass it, another limitation may stop you. Apple's adapter still looks like Thunderbolt to the Mac, so if your application won't work with Thunderbolt, the adapter may be useless to you.
One documented failure mode is running Microsoft Windows under Boot Camp interfacing to non-hard-disk FireWire devices. Windows works fine under Boot Camp with external FireWire disk attached via Apple's adapter, but it does not have Thunderbolt drivers compatible with Apple's adapter for non-hard-disk FireWire devices. A workaround is to run your Windows application in a Mac OS X-resident hypervisor, such as Parallels or VMware Fusion, both of which work with Apple's adapter.
There may be similar compatibility issues with non-disk FireWire devices such as scanners, cameras, and music processing gear. If the controlling application doesn't support Thunderbolt, it may not work with the adapter.
Device-specific solutions: Enclosures and intermediaries If you determine that Apple's "universal" adapter solution won't work for you, you'll have to move on to device-specific solutions.
An easy, although somewhat labor-intensive, workaround for external hard drives is to change to a new enclosure that supports either Thunderbolt or USB 3.0, such as New Technology's $99 MiniStack enclosure, which has both USB 3.0 and FireWire interfaces. For hard drive transfer speeds, USB 3.0 and FireWire 800 have equivalent performance. You can purchase the enclosure now to future-proof your drive investment.
What if you have a bevy of external FireWire hard drives? That situation is not uncommon with video professionals. If you don't need portability, you may be able to use a Gigabit Ethernet NAS device, assuming the device supports JBOD (just a bunch of disk) RAID technology, such as Synology's $429 four-bay DS413j. Depending on the NAS device's firmware, you may be able to just insert your drives and access them individually over Ethernet. Alternatively, you may have to buy one initial hard drive to migrate your existing drives one at a time into the NAS, adding your drives to the NAS array as you go.