USB1.1's maximum bandwidth is 12 Mbit/s, or 1.5MB per second. About the same speed as slow ethernet or Airport. FireWire, on the other hand, is 400Mbit/s, or around 50MB per second. Much faster. And there's USB2, which has a maximum bandwidth of 480Mbit/s, but the sustained transfer rate is only about 200Mbit/s. However, USB2 can be used (at slower speed, of course) on a USB1.1 equipped computer.
It can get confusing because some buses (i.e. SCSI) measure their throughput in MB (megabytes) per second (Ultra160, Ultra320 have 160MB and 320MB per second, respectively), whereas others (like USB, FireWire, and Ethernet) measure it in Mbit (Megabits) per second. There are 8 bits to a byte, so there are 8 megabits to a megabyte.
I'd be concerned about a hard drive based MP3 player that uses only USB1.1, because you'd have to spend so much time transferring the music that you will spin the hard drive for a lot longer, causing a shorter life. USB1.1 works fine for those little Rio players that take compact flash cards, because their capacity is only 32-128MB usually.
Incidentally, I talked to someone in my Calc class today who has an MD player. He really likes it. He tells me that the player itself can get MP3s from his computer and then burn them at 32x speed onto his little rewriteable discs, which can store up to 5 hours of music (depending on the bitrate; he says there's no real difference between the highest quality and the medium setting, though he's not tried the lowest one yet).
Okay, so this post wasn't too well-written... but I hope you got something useful from it!