I never actually had a lot of fun with Duke Nukem 3d.
I liked the design of the levels and stuff like that, but the actual battle wasn't fun. I don't know how to describe it exactly, but it wasn't as challenging and rewarding as Quake. In D3d if felt like the enemies were just there to fill space. Evading enemy projectiles or the enemies themselves, how they reacted to your ammunition etc worked much better in Quake, which has much better action.
As for DNF...
I tried it out, and I didn't like it. I wasn't even prepared to not like it, I lowered my standards to a realistic level and I still expected more. The battle is just crappy and boring. Maybe I'll play a bit more and see if it gets better.
Also, maybe it's just me, but a lot of the interactivity that comes in games like this (such as being able to pick up poo and throw it around) is totally unnecessary and gimmicky when it doesn't add anything to the gameplay.
I finished it today. The problem with the game is not that over 12 years of development are unjustified, but because it's, at best, a mediocre gameplay experience. There weren't any memorable moments... Winning fights wasn't satisfying and the fights themselves were repetitive and challenge-less. Well, I admit I did have fun eventually. I even chuckled at a few jokes .
I've not seen this in any review I've read, but I have noticed elements of conflicted design. The whole thing about recharging health seems to have been added much later in the development. For example, in some levels there were things like electricity jumping that you had to time to pass through... Getting hit by electricity costed health, sure, but why should I care if I'm still going to recharge? Note that there weren't any enemies whenever you had to go through things like this. These kinds of damaging obstacles were boring because they were there just to be there.
Being able to carry only two weapons is more annoying than I thought, but most of the time I was carrying the shotgun and the ripper because they are the only weapons you actually need in this game (aside from the RPG/Devastator which are the only weapons that can be used against bosses). But you rarely get a chance with the shrinker or the freeze ray. The fact that I couldn't save up the 10 shrink bullets for later because I could only carry 2 weapons made the weapon seem like it was there just for nostalgia value. Same goes for the freeze ray. It actually recharges but it's too short and depletes too early for large fights, so it's actually quite useless.
I did find the shrink parts interesting. It was probably the only unique thing about the game, even though it also existed in D3d. Fighting behind cans and sauce jars or using toasters as a means to jump and reach a high shelf made me wish the whole game was like that.
To sum up, the game felt rushed (!) and that it wanted to be much bigger. It's lacking what the developers themselves actually wanted, but they wrapped it up in a lesser package and shipped it away. However, it does provide some fun and it's worth playing just for nostalgia and curiosity.