I have been using a Marantz PMD-670 for over a year and the internal mic is
decent and works very well in many situations. Typically, I might do an
interview seated round a table/desk in an office or someone's home with the
recorder placed on the table/desk between myself and the interviewee and
within 2 or 3 feet of each of us. As the recorder is solid state the
internal mic doesn't pick up any motor or other noise from the recorder. I
think distance and background noise are the main issues here. If there is a
lot of ambient noise, say one was sitting at a picnic table in a park next
to a busy road one would definitely benefit from a clip on mic so that you'd
get a mic within 6 to 12 inches of the interviewee's mouth and maybe a
similar mic on the interviewer. Or if you were doing a focus group or
recording a meeting and it was impossible to get the recorder within a
reasonable distance of everyone then two or more mics (maybe PZMs) spaced
around the table would help. That said, I've used the Marantz with the
internal mic alone in both the above type of situations and and had usable
(if not great) results.

I guessing that the mics (there are two) in the PMD-660 are the same or
similar to the single internal mic in the PMD-670. The reports I'm seen
suggest that the Edirol's R-1 mics would also quite useable for this type of
application.

My comments on Hi-MD aren't based on experience but on comments I'm read on
the minidisc.org forums. I own two earlier MD recorders which are now
gathering dust. My two cents: MD has always been and potentially still is a
great recording technology that has been technologivally limited because
Sony has concerns about digital copying. That's fine but there are lots of
people working in academia, business, education, law, government,
broadcasting, entertainment etc. who need to make original recordings and
copy, transfer, and manipulate those recordings as they please in ways that
are quick and reliable without having to spend a small fortune on something
like a Sound Devices 722. Marantz is clearly aiming at those markets (check
out their website) while Sony is clearly more concerned with selling
products into the market for portable music players (although Apple seems to
have whipped them good and well in that market).

That said I still think MD has some advantages at the moment. One obvious
one is that someone on a tight budget, such as a student, may find a $250
Hi-MD recorder more appealing than $400-$500 for a solid state recorder.

I haven't used the little solid state memo recorders Duncan mentions. A
couple of years ago they had pretty minimal audio quality. More recent
models have much higher sampling rates, record at much higher bitrates using
much better and more widely used audio codecs (e.g. WMA). The Olympus DM-10,
DM-20, and DS-2200 all look interesting. And you can easily upload to
computer. They may be another cheap alternative worth considering. In the US
I think the DM-10 can be found for as little as $150.

One last comment on reliability and ergonomics. The Marantz recorders are
designed for field recording. They will take more abuse than the others and
they have big, well-positioned controls and displays that allow you to do
the job with a minimum of messing around. When I was using a Sharp SR-60 (an
MD recorder) I used to have a seven-step checklist of buttons I had to push
(tiny little buttons with cryptic labels) to set the recorder up after I'd
turned on the power. All that is done in advance with the Marantz. Now I
just stick the Marantz on the table flick a decent-sized power button, flick
a large (thumb-sized) red record button and that's it. It's fast, it's
reliable, and I don't have to think about it.