I use a Radium-61 as a USB-based midi controller. It also supports standard midi cable interface also. It has 8 knobs, 8 sliders, and many other controls that share keys. I paid $260 for it, and, although it feels a little "cheap", it actually has proved to be pretty sturdy, works very well, and got me off to a good start with soft-synths. Obviously there are better controllers out there, but this one was well worth the $260.
