Hey Mike, while I haven't used this specific model, I have used the OWC Mercury Rack Pro, which I'm pretty sure uses the same chipset.
I didn't get the kind of Firewire write performance I expected to -- only about 40MB/s. Read performance was about what they advertised at 80MB/s. eSATA performance was excellent and right at their benchmarks. It's been reliable for the last couple years I've owned it, and I know a few people in town who have run Mercury Racks as backup storage for servers the last few years without incident. They're at a great price point.
So, the simple answer is that this box can probably handle your workload fine if you're maxing out with ProResHQ. And, since you're limited to firewire, all you can really expect out of any RAID box is around 80MB/s.
However, there are some RAIDs out there that are more targeted at video editors with a good reputation for service and reliability. They typically use higher-quality disk drives. G-Technology comes to mind: http://g-technology.com/products/g-speed-q.cfm (If you click their banner ad on the COW, I believe you can get a discount.)
Can I really say that the price difference is worth it? Not sure. If you're a pro and this RAID will be your livelihood, I'd say go with the more recognized and supported RAID with the better drives. If you're looking for something a little less serious, I'd say the OWC is a decent option, and mine has been working fine (with the small stipulation on firewire write performance).