I just played some dukematch and had a lot of fun. Either the netcode or the server I was on was pretty gay though, as people would frequently teleport behind me after i'd been drilling them for a full 2-3 seconds and wondering why they weren't dead.
Mics are set to always-open if you have one, for some reason. The very first match I played I had some awful feedback so I turned voip volume off, but I later turned it back on to hear people giggling and moaning during the match. It was pretty neat.
Two of the deathmatch levels I played really stood out--they converted e1m1 from duke3d into a map, and the scene in duke burger when you're tiny is also in there. It reminded me a lot of SiN's deathmatch in the oddness of the proceedings, but things seemed to be decently balanced when lag wasn't dicking me over.
Duke also has tons of lines that you only hear in dukematch and he made me lol a few times. I don't recall him repeating any lines, either.
Re: melvin; I don't know what has some of these reviewers so emotionally-charged. It's pretty clear to me that none of them actually enjoyed Duke3D and are seeing the other negative reviews and trying to dogpile on the title.
Finishing the game unlocks a bunch of extra content, including all of the previous DNF trailers, a history of DNF in timeline format, pictures of the game throughout development, concept art, pictures of the offices, a soundboard, and a harder game mode that you have to play through again to get the 'real' ending.