In the summer of 1975 I did a fair amount of documentary recording with a Nakamichi 550 in Europe. The mike inputs are very low-noise but unbalanced. Interestingly they use TRS sockets, i.e. conceivably one could build miniature input transformers such as Beyers or Lundahls into the recorder without needing to change the sockets for use as balanced inputs. There is no form of microphone powering in the deck. The wide-range metering was a nice feature, the limiter was very effective, and overall the deck was quite reliable.
The center-channel mike input has to be used carefully--Nakamichi's published three-mike theory really makes sense only if you have three recording channels and three loudspeakers for playback--but I suppose the hardware (if you are very lucky in setting your relative record levels) can be adapted to other three-microphone approaches such as Decca Tree. I used it for feeding the signals from a radio mike into the center while making an overall stereo room recording with the left and right inputs. The results varied because again, you have to hit the center channel level just right, or it either dominates the recording (and makes it essentially mono) or else it just doesn't add what you hope it will add.
The two problems I had with the recorder were a certain low-level leakage of a high-frequency tone in playback--as I recall, this wasn't actually recorded on the tape, but was audible over headphones while recording--and the general difficulty of setting the bias and EQ accurately, which of course is a problem with two-head recorders generally. Also, the 550 is from the generation of deck that uses Nakamichi's old, non-standard EQ for tapes other than Type I, which makes any of those tapes recorded on their decks sound a little dull when played back on any other brand of cassette deck. (Conversely, this made their decks sound better than any other brand you compared them to in hi-fi showrooms, because they played tapes back a little brighter.)
The 550 unfortunately doesn't have Nakamichi's excellent dual-capstan mechanism, and of course a combined record/playback head can never be as good as two separate heads, since there are directly opposing design criteria for those two functions. But that was the compromise they made for the sake of acceptable battery life.
--best regards