I have four Denon units bought new circa 1990 (CD player, cassette, tuner and integrated surround amp), and much later I added two more secondhand units (MD and LD players) just to "complete" the set.
The only problem I've ever had with one of these is the power on/off switch on the HT amp. That seems to be an occasional weakness in these, as I've seen two other secondhand units that had the same problem. The little catch that closes the flip-down panel on a secondhand unit I had wasn't sticking shut properly, but it worked fine after a squirt of cleaner.
The other potential weakness with Denon units is that (I've been told by repair techs) some of their ICs are getting hard to replace if they ever go bad, but fortunately none of mine have so far. It's a long-term worry that is shared with ALL modern IC-circuit gear.
Quality? Excellent for the time it was made and the price.
Sound? -Again, excellent... as long as your speakers are good!
Looks? -With golden-champagne-colored aluminum fronts and wooden panels on the sides, these have a sophisticated look that blends especially well with antiques, wooden stuff and traditional interiors. I'd describe it as a "subtle, two-martini appearance".
My stuff isn't the very best Denon ever made, it's middle-of-the-line to upper-middle... but it's good stuff (compared to the competition) that I plan to keep, and would buy again. The "Special Limited Edition" CD Player competes well sonically with some at 2 or 3 times the price. Some of their high-end stuff that I've heard is among the best currently made by any big (non-boutique) manufacturer. Not as solidly built as the late-seventies stuff, but good for what it is. Just my impressions as a long-term Denon owner/user.