For a long time IEEE 1394 firewire was faster than usb (1.1) and a lot of camcorders and other high data rate devices used firewire. Sony calls it i.link (kinda funny that some one other that Mac uses "i" in a name)
Anyway this is from memory but
usb 1.1 had 14 Mb/s
Firewire 400 had 400 Mb/s
usb 2.0 had 480 Mb/s
Firewire 800 had 800 MB/s (hardly any devices yet that use this)
Again, those numbers may be off --- I use to work at an office supply store and this question came up often --- those numbers were from memory (don't wail on me if i'm off)
In answer to your question, if you don't plan on video editing (or have a new usb 2.0 camcorder) I wouldn't worry about it. If you want to be able to accept all forms of imput and have the $25 bucks, go for it. I know I use it but both my sony camcorders use this (but most newer ones have the option to go usb 2.0)