I have played DNF for two days now. My opinions of it are a bit mixed but I have yet to finish it.
Quality of assets is slightly varied, which is quite understandable given its development history. But is it ugly? No, I don't think so. There are a few places where textures for example were clearly sub-par. My biggest gripe graphics-wise is that everything has plastic feel to them most of the time. So bottom line, graphics are pretty much average. As for sounds department, I think sounds could have had a bit more depth and sharpness to them. Sometimes the audio environment just kinda feels dull. Music is what you'd expect from a DN game, but I think the soundtrack is a bit lacking in variety and gets monotonous after a while. So again, average.
I think DNF is sorta fun to play. At first it is odd to play shrunk Duke like if it was DN1 or 2 turned 3D. Or drive RC car while shrunk or monster truck in various places. It felt out of place but in the end it served as good distraction of the usual shoot 'em up play. Controversial themes like Duke's one-liners, the girls, etc do give a good chuckle every now and then. Difficulty seems to be quite fine, the medium difficulty isn't too hard or too easy.
Guns pack enough punch not to feel peashooters. I did notice though that sometimes it feels as if DNF uses armor.tbl, pigcops can sometimes take a lot of punishment but sometimes they die very fast for example. One time its enough to kill a pig with few slugs from the Ripper, another time you need to empty a whole clip into one to drop it. It's odd. The two-gun limit has been mentioned every second post and I tend to agree. While guns are almost always laying around waiting to be picked up, it just doesn't feel like Duke not to be able to carry a whole arsenal with you. There were a few times when I ran out of ammo at which point Duke automatically switches to his next weapon. If that happens to be RPG or Devastator and you're still pressing the trigger, well, let's say you may end up in small bits before you notice it. I really wish Duke's kick would have been standard melee attack.
There's no saving game manually either, it's all checkpoints. Though checkpoints are basically littered around after every fourth corner so it's not really a problem. What is a problem though that weapons dropped by killed monsters vanish when loading a checkpoint.
What was shown in the playable demo isn't much to speak of. The mine level is actually really boring compared to most other levels and I don't get why they decided to show off that level in a demo, there would have been much better demo material too.
My verdict so far is that while DNF doesn't suck, it is still fun enough to play that it is strong in the average class. It could have been better and I actually hope 2K Games and Gearbox create a new Duke game from scratch that can live up to modern standards while still being a Duke. Is it worth full price? Nah. I'd say it's totally worth buying it for bargain prices though.