I just bought an MCX8000 in December as a Christmas present to myself and couldn’t be happier! I bought used on eBay for $650 USD after taxes and shipping.
I currently use it as a backup to my SC5000/X1800 (have had some odd sound issues with those that I haven’t deep dived just yet, but I digress) due to portability and it’s just an all around cool controller.
As has mentioned, buying used is always tough. My unit (as well as the SC setup which was also used) appeared in incredible shape and I spoke with the seller to get more details as needed. If buying through in-person or classifieds, always try before you buy! It’s relatively easy to test all the bells and whistles on the MCX (I did as much when I received it).
For the known issues, I (knock on wood) have not encountered any just yet. I’ve ran multiple 4 hour continuous streams and records with it and it’s worked flawlessly.
All in all, like any used product, just do your due diligence and you should be okay. The MCX is a sweet controller that I actually enjoy slightly more than the Prime 4 (I’m not a big fan of single screen stacked waveforms like the Prime 2/4 or RX2, much prefer stand-alone by the decks) and nearly 1/3 of the cost, it’s an absolute steal!