Well, I don't own the steam version of the game. But if the steam store page for the game is to be believed then the game has both "enhanced" and "original" versions. So a weird winport and the dosbox version included as an extra.

Taken from the steam store page:
"Duke Nukem 3D: Megaton Edition includes:
Duke Nukem 3D: Atomic Edition - All four officially released episodes of the iconic shooter with carefully restored visuals, timeless gameplay, and challenging Steam achievements. (OpenGL)

Duke Caribbean: Life's a Beach - Grab your water gun and sunscreen as Duke battles vacationing aliens in beach side hotels, tropical lagoons and even a cruise ship! (OpenGL)

Duke It Out in D.C. - Duke takes his special brand of politics to Washington D.C. to turn back the alien invasion set against the White House, the Lincoln Memorial, and even FBI headquarters. (OpenGL)

Duke: Nuclear Winter - Take a brutal walk through this winter wonderland as Duke gets into the holiday spirit by bringing the battle to the North Pole! (OpenGL)

Duke Nukem 3D Classic - The classic version of Duke Nukem 3D in all it's original glory. Hail to the King, baby! (DOSBox)

"
Hmm, it also seems that the expansions are only for the OpenGL version?

I think the port is JFduke but they messed up parts of the code or at least got rid of things that they shouldn't have.

EDIT: I've also reported these issues and a developer has complied with an acknowledgement to at least two of the problems so I don't really know if they'll fix ALL of these issues.

Lets hope they fix these issues before the multiplayer update.

xduke is still the way to go for me... an executable, decent multiplayer support and proper UI scaling.

I tried it (before buying and regret for the rest of my life) and god, weird mouse acceleration you can't turn off. Guess the MP side of this is gonna be good on steamworks but they really need to fix the mouse support.

I won´t buy it. I don´t like steam really

Already own two physical copies of the game...would be worth it to have one really good version of the game, I'm just not convinced that this will be it.

Does it include all the GRP files from the expansions?

Yes it does. In the gameroot folder.

Cool, damn cool. Did this come out of nowhere? It wasn't on my radar, and I like to think my radar can detect the stealthiest of releases.

Pretty much came out of nowhere, yeah.

Yesterday I found out they patched it last thursday and fixed that issue, I fired it up and went through Hollywood Holocaust and Duke It Out In D.C. (that was one awesome expansion) with no trouble. It's a pretty cool release if you ask me. I only wish one day they'll manage to port the Plug N Play episode exclusive to the PSX version on it.

Yup, can confirm that it is all fixed for me. Been enjoying the trip down memory lane.
I have the Good Old Games version to play on my laptop as well (its a company laptop so its not reaqlly meant for games). Both versions are worth buyoing and are quite cheap. I still have the atomic edition disc as well lol.

This evening I got this Megaton Edition (50% off coupon, thanks for that Steam). It actually runs quite well, really, even if it isn't eDuke32.

There are a some points to note, though:

1) The mouse seems too sensitive by default, but dropping it down quite low made it usable for me.

2) The volume (as mentioned here) applies to both music and sound effects at the same time, no separate sliders for each.

3) The default brightness, even set very low, is brighter than expected. It is okay for me, but for anyone wanting to darken the game, there is very little room to do so, meaning you'd have to find alternative ways of doing this, such as monitor brightness. However, who is to say my monitor isn't too bright (though I don't think I ever altered my monitor's default brightness anyway).

4) The port used actually launches the games from a Steam menu, meaning you don't get a port front-end, it just goes straight into the chosen game/expansion. Guess you can get around this with command line options if you want to load custom levels, but you'd be better off just grabbing the GRP and CON files and running them in another downloaded port instead.

5) If you run the original game in classic mode it uses DOSBox. However, it is an older DOSBox, v0.72 rather than 0.74. Seems odd to use 0.72 considering how long 0.74 has been available.

Well, I can quicksave and quickload fine, using the default keys of F6 and F9 respectively. You can change these from the key settings menu.

As for HUD sizing, well, it is either statusbar, brief or none, it isn't a scalable statusbar, so once you've chosen your setting you probably won't want to keep changing it, so I've no issue with that being set from within the menu only.