At the moment it seems that about half the USB flash devices on sale are listed as “Vista ReadyBoost Ready“. I recently bought an 8G USB device that I returned because it could only sustain 4MB/s writes (not much fun for backing up 4G+ of data). So I’ve been wondering whether I should get a ReadyBoostReady device.
ReadyBoostReady only means that it is faster than other devices in some ways, it has no inherent feature difference. After some searching I found a FAQ about ReadyBoost  which explains what it requires.
It seems that ReadyBoost needs “2.5MB/sec throughput for 4K random reads and 1.75MB/sec throughput for 512K random writes“, which isn’t really a lot. About 5-6 years ago I was running some machines with 4 disk RAID-5 arrays that could sustain 3MB/s writes for significantly smaller block sizes (maybe 12K), given that random access is something that disks are really bad at (physical movement of the heads is required) and something that flash is good at) it gives an indication of how low the performance bar is for ReadyBoost.
I’m sure that a device which meets the minimum specs would do some good if you have a single disk that’s not overly fast. But if you have a decent RAID array for swap then I doubt that the minimum requirements for ReadyBoost would give a benefit.
As for doing bulk transfers, it seems that while ReadyBoost Ready devices will have consistent good performance across all their storage (apparently some devices perform better for the first blocks so that people who use FAT based filesystems can get good performance from their FAT) they won’t necessarily have particularly good performance for bulk IO (1.75MB/s is pitiful when you want to copy DVD images).
There are some USB flash devices that are marketted as having high performance and which are supposed to sustain >20MB/s, but I’m unlikely to find them going cheap at my local electronics store. I had hoped that ReadyBoost would be demanding enough that the ReadyBoostReady devices (which aren’t the cheapest flash devices on sale but often aren’t too expensive) would satisfy my requirements.