It took me two days to beat the game on Normal (The easiest of three difficulty settings). Really not a lot more than 5 hours playtime, for every level and 100% secrets in all levels (Though I can't seem to get 100% kills in the Forge... Anyone else having that problem?).
Things to take into account:
-I'm a Doom RPG veteran, so I know my way around O&E. Cell phone gaming is a good platform because of stuff like GameLoft, and id's Doom RPG/O&E
-There are two difficulties above Normal that I will likely play through as well.
-At any given time in my Normal mode run through, I had 20+ of EACH potion. There is a lot of room for sane difficulty increase for the Hard difficulty, and probably a lot of room for "Every combat a puzzle" sort of insane difficulty in Nightmare.
-If you 100% killed and 100% secret'd in Doom RPG, you'd get a secret website you could go to. I imagine there might be something unlocked for 100% secrets and kills in this, but damn it all if I can't seem to find two enemies in the Forge.
-You can also collect scrolls that unlock gallery art. O&E isn't exactly high fantasy art, but it is more interesting than Oblivion or whatever. Still, these gallery pictures are uniformly uninteresting. They're not offensive... like the worthless pencil drawn blobs in Touch the Dead, and you'll look at them at least once just to go "Hmmm", but they aren't cool.
-This is still a port of a cell phone game. An beautiful port, but a port all the same. This is light entertainment as a result, not a huge epic. I won't begrudge CAGs who hold out for a price drop, but Doom RPG fanboys who never had a cell phone that could run O&E owe it to themselves to get their O&E DS on.