I'm just starting to use a PMD660 and while the pre's are not the standout feature of the device, they aren't hideously bad either. Oade Brothers offers a noise and distortion reduction mod for $125, BTW. Similar to what they do for the PMD670. The internal mics, OTOH, really do sound like ass for music. Keep in mind that this box was destined for the field recording of interviews, conferences, etc... and not really designed for audiophile applications. Anything that I would be recording on this will end up getting washed through my DAW anyhow, so I'm not going to freak if the raw audio isn't blowing my socks off.
It was a tough choice between the R1 and the 660, but I sold off my MiniDisc rig last year along with my SoundProfessionals binaural mic and preamp rig, so I liked the XLR inputs on the 660. That lets me use mics that I currently own instead of having to buy new mics.
The device that sounds as good as my MOTU896HD, is the size of a paperback, and runs for 12 hours on a set of AAs hasn't been invented yet. Since the cash for a Nagra wasn't falling from the sky, the PMD660 seemd like the next best thing.
USB data transfer
CF memory
built in mics and a line in
XLRs with Phantom power
Battery operation
compact size
I'll be giving it a good workout over the next month and I'll have a better idea of how "acceptable" the audio is by this time next month.
Pete