Cassette decks. How good can it get?

Unless one has special recordings on cassette already that cannot be had elsewhere, I would recommend to not  bother with cassette. 
 

I still keep my old Yamaha cassette deck around for occasional use.  If I play a tape it is only once and is digitized and added to my digital music library.   Same with old  vhs hifi recordings I still have which provided superior sound quality with a good player in its day. 

The original poster was asking how good can cassette decks get. To be able to answer this question, one has to have hands-on experience with several (if not ALL) of the top decks that have been restored, -or still operate to factory spec, and that are properly calibrated to the tape being recorded on. Understanding different tape formulations and how Hot a signal each tape can take has to be well-understood, and the blank media must not suffer from dropouts, molding or shedding. If you own a Dragon, or "an old yamaha deck" or a "BX300 that is not working", I don’t think you can provide a valid input to this question, let alone recommend or conclude whether to "bother or not to bother with the format". Also, repeating that "the 1000zxl is the ultimate deck" (most likely without owning one) is just not enough (-or true).

Both the Dragon and the 1000ZXL are boring sounding decks with dynamics, scale and slam simply gone out the window. They make lifeless and small-scale sounding recordings, but to their credit, they’re fairly good playback decks (whenever they’re up and running instead of being serviced). Nakamichi themselves chose the ZX-9 for their studios and master recordings, classifying it as their highest performing model. The ZX9 is great (and outperforms a Dragon or a 1000), but to my ears the CR-7 and a 682 are even better. Especially if you record on a CR-7a, and play it back on a 682. But that’s only the Nakamichi brand.

Anyway, in short: the very best sound quality that was ever possible with the cassette format (to my ears), was provided by the Tandberg 911 and 3014 (whenever they were not being serviced but actually worked), the Studer A721, the Technics 9900, and the NAD 6300. These walk all over anything Nakamichi made, these can make the recording sound superior to the source, they are utterly musical, bold, clean, dynamic and engaging.

Record with an NAD 6300 and play it back on a Tandberg - and you will find that the sound quality is now in the territory of a 1/4" Nagra reel recorder.

So to answer the original question: in my opinion, cassette can get very, VERY Good......

From my arsenal of over 100 decks (over the years), I’ve only kept the Technics 9900, the Studer A721, a CR-7a, and two NAD 6300’s. They still serve up immense fun.

Just to follow up to Fonoman and all that excellent advice.  I ended up finding a mint condition NAD 6300.  It arrived broken and I had to send to my tech.  He finally figured out there was a piece of hair wrapped around something that was causing the issues.  It's now working great and is a joy to use.  I love the sound and to my ear it's a significant upgrade to the NAK BX300.  Thanks for the advice and I'm collecting more tapes than ever.  

I only have a few older OG and a few newer tapes and bought a TEAK V-7000 off eBay. Sound fine to me, but I have no idea about either tell-tale signs of issues, or if any regular maintenance is required, say degaussing heads. I also remember some special cleaning tapes. Thanks for any wisdom.