Here's a taste: the best CompactFlash cards on the market now are capable of real-world throughput between about 15MB-17MB/second in the best shipping readers we've tried. By comparison, the SanDisk Extreme IV 2GB, when inserted in an Extreme FireWire Reader, tops out at a whopping 38.6MB/second, with the Extreme IV 4GB weighing in at 38.4MB/second. These aren't synthetic benchmarks, but the actual speediness of Extreme IV when moving JPEG and RAW picture files to a Power Mac G5 here.
Assuming we can expect that performance in other systems, that is quite impressive. Of course, with whopping performance comes whopping price, and the 8GB model will run you as much as $640, plus the cost of the high-speed FireWire reader. With an 8GB flash module now on the horizon, it's no question at all as to whether or not we'll see HDD alternatives in the near future. Let's just hope the cost comes down before then.