The DDM is a great mixer.. Absolutely worth the money. I've just seen 2 die now, so I would keep a backup on site.. I've had nothing but good luck with behringer over the years, and I continue to recommend behringer mixers over mackie's cfx or dfx series any day of the week.
Now for denon... I'm a huge denon fan! they can do little wrong in my eyes.... except for mixers...
I've used or owned several denon mixers over the years, as I've either bought them, or had loaners to demo or whatever.
They are noisy.. They have horrible eq curves... They're cheaply made, the mic pre-amps are absolutely crap, and they make audio sound worse than the source. We ran an iPod through the dn-x900 in Atlantic city at dj times, and then directly into the qsc k12, and all present agreed that the mixer colored the sound negatively. (this was obviously not the denon booth)
Like I said, they make great cd players, but you'll rarely see denon mixers in the wild in installations where quality matters. Sports bars and places where they cut corners on the sound system yes, but not at big clubs...
Put it this way... It's the most important component in your rig second to the audio source. With a few exceptions from ran, and Allen &heath, there are few mixers under a grand that are really nice mixers. I used to argue this point till I was blue in the face... I had a dn-x900, and I ended up buying the almost $3000 Allen & Heath xone464 mixer after hearing the difference.
My arsenal is all pioneer 5000's, Allen & Heath zones, and ran mp series mixers now.
Denon may be a better name than Numark or Gemini, but the performance of their mixers is in some cases worse than a Numark or Gemini.
That's real world 6 night a week actual usage review opinion right there.. I wouldn't put a DN-x series mixer back in my rig if Denon gave it to me for free, and paid me to use it.