SP campaign if you go for all secrets etc lasts about 18 hrs on "hurt me plenty" or "normal mode". I think hurt me plenty mode is rather easy tho, except for one boss fight, so I wish I would have plaid the game in the hardest possible start off difficulty setting (there are two additional ones to be unlocked)... that is all I regret so far, campaign was easy in Hurt me plenty, too easy. The only con here is that you basically just play in a big ass map, rather than Half life playthrough of going through many new areas, and the mobs spawn if you walk to certain areas, so you will see a lot of repetitive scenery. Also, the dark hallways are gone, this doom SP definitely does not give you the fear factor like it did in the previous Doom from quite some years ago. I am a cut-scene lover, so almost none here, which was a bit of a disappointment for a big SP fan. The sound keeps you pumped tho when the fights start, but the perks that you pick up like berserk, well, to me they are a sandwich short of a picnic... no extra adrenaline rush music...SP for me was a 7.5 / 10.
MP is a blast. I hate MP games, but this here is fun if you got quick reflexes. This is a huge surprise to me... also, the noobs that think all they gota do is jump and use rocket launcher all the time get owned hardcore, so don't think just because you got a big gun you will farm people, game mechanics basically work in a way where if you are a noob/boring/lame player, you are done for, thus this makes it fun for non-MP fan players...MP for me so far, 10/10 absolutely...
If I was to rate this, 8.5i but I can't here so I round it up to a 9.