USB2 maxes out at around 30 megabytes per second when transferring to/from external hard drives, USB3 goes as fast as the hard drive, I regularly see 90-100 MB/s transfers. The USB3 interface itself is about 10x faster than USB2.
I have a little Seagate 1.5TB GoFlex drive that I hook to my laptop and edit via USB3. I also have a USB3 dock that I use on my desktop machines, it is every bit as fast as my eSATA dock.
Yup, I because I'm running out of HardDrive space on my laptop, I recently purchased a 1.5TB Seagate GoFlex plus USB 3.0 ExpressCard. I've moved projects directly to this drive and seem to take about 25% performance hit, but it seems to work just fine.