I have an LX-5 and 581.  I've heard a few at used stereo shops.  For my money, the 581 is the nicest sounding and plain coolest Nak I've ever used.  It has nice soft touch buttons.  It operates VERY smoothly.  Alas, my 581, which is  GIGANTIC wouldn't fit in my rack, only on the top where my table goes.  So, when the motor in my LX-5 went caput, I took the motor out of my 581 and the brass fly-wheels out and put them in my LX-5.  The LX-5 is fantastic.  One thing to keep in mind is that the middling decks, like the 581 and the LX-5, with a reputation for excellent sound have long ended there devaluation are going up in value as more and more hipsters take on the cassette deck.  So, get one and get one quick.  And while the ZX-7 and Dragon are nice, you'll get smoother operation and keepp much more cash in your pocket to go with a late 70's - early 80's belt drive logic controlled gear than the clicky clanky computer controlled gear drive stuff.  The older decks also have what appear to be brass flywheels in the transport which are noticibly heavier and better balanced, which I would guess contribute to the pitch stability and smooth operation of the pre 80's decks.

Good luck!

 I had A 481 for many years and loved it, but the mechanical parts finally gave way to wear and parts became impossible to get.

Buying one is like buying a very old obsolete car; old technology and worn-out parts. I can't imagine it, except as a curiosity for your "old hi-fi" museum.

Modern digital recording technology gives much better sound quality and lower noise levels. Cassette technology became obsolete for a lot of good reasons...duhhhhh.

Although I own three (IMO) very good cassette decks (LX-5, ZX-7 and Teac V-7010) I wouldn't recommend one. Unless you have an investment in cassette's, either pre or self recorded, there's no real reason for owning one. Of course, if you simply want one just to have it, any of Nakamichi's three head decks should satisfy. The Teac I picked up a couple of years ago IMO looks better than either of my Nak's and sounds damn near as good. 

I have a Harman Kardon TD4400 cassette deck which was designed to transcribe CDs no less. A very good deck. If you can find a TD4200, TD4400 TD4500, TD4600 or TD4800 in good shape and a reasonable price you should consider it. The TD4800 definitely rivals some Nakamichi decks and is about 10 years newer.

IMO only three head decks are worth bothering with. That pretty much means a "vintage" deck.

There are a half dozen or so new decks on the market. AFAIK all of them are two head dual deck machines. Back when cassette decks were popular dual well decks were not considered top tier. Of course with no alternative what can you do?

BTW: Vinyl applies to the singular and plural.