This Vantec-branded enclosure houses a Seagate Barracuda 7200.12 500GB hard drive whose peak transfer rates aren't anywhere close to 600MB/s.
USB 3.0 easily outpaces its predecessor, offering up to 3.5 times the throughput of the old Hi-Speed standard in HD Tach's sustained transfer rate tests. The Barracuda 7200.12 isn't capable of transfer rates much higher than 120MB/s, though.
Of course, we also saw a similar jump in performance when moving the hard drive over to its native Serial ATA interface. eSATA hasn't really caught on, I suspect because initial implementations required an auxiliary power cable. However, hybrid eSATA/USB ports are slowly populating motherboards and notebooks, and they may offer a better interim solution until USB 3.0 sees widespread adoption and more robust implementations.