Unfortunately, I know of no tool that does the job well. I have an SP-808, and know how to use pretty much all of it, though I must confess I don't get to very often. It has 4 voice polyphony which would be a limitation if I was making all my sound with it, but isn't to bad when you are using it as accompanyment, somewhat clunky interface, but at least it has one. A teensy bit more forethought on Roland's part would have made this machine absolutely killer. Too bad.
It's good for playing back >really< long samples - like entire songs, while playing loops and other stuff over the top, impromptu. I have mine set up with long ambient backing tracks,
It uses zip disks, which is good and bad; it allows it up to 45 minutes of sample time at 44.1k and 60 at 32 (which sounds very good), but you're using zip disks; not the most reliable of medium. The best part of the zip disks is that you can transfer loops on to them from your pc using a freebie applet that Roland makes.
If 45 minutes isn't enough for you, it uses any IDE style drives, or so I've been lead to beleive. The gear whore in me has been lusting after a pair of ORB drives (one to put in the unit, one to put in my PC for sample transfer), which are the only large capacity IDE drives I can seem to find. Does anyone know of any other 3 1/2" IDE removable drives? Not that I think I'll be getting them soon.
I've heard a lot of mixed reveiws about the su700, mostly bad, but some good. It combines the now-legendarily slow Yamaha SCSI connection with 'send it back to the factory' level hardware/OS issues. Still, I read reviews at www.sonic-state.com written by people on their third and fourth replacement, so there must be something good to them. Waiting fifteen minutes to load 60 megs of samples isn't my cup of tea though.