The Ultimate DOOM is the definitive edition, featuring the 3 initial episodes and the later addition, “Thy Flesh consumed”.
JohnnyNoArms: Is this stuff included with the Doom that comes with Doom 3 BFG edition?
Vampyr78 gave link to the full article with technical details. I'll give you a short resume on that.
Pros: * Whole new episode 'No Rest for the Living' supplied as a separate WAD file. A really good one. Maps of high quality and of transitional between DOOM I and DOOM II feel and look. This alone justifies the purchase.
Cons: * New level of censorship not present in the original releases.
* Missing features and visuals as compared with the original releases.
* Additional map33 for DOOM II, while being positioned as a bonus, is neither new nor good. And even is not accessible without third-party source ports due to a bug.
So, if you want the original Ultimate DOOM or DOOM II I'd recommend to purchase them separately as a classic versions.
'No Rest for the Living' worth buying and playing but the only official and legal way to obtain it is to purchase a pack which includes as an extra
inferior versions of Ultimate DOOM, DOOM II and DOOM 3.