Considering the time you spend gaming? Yes, its singleplayer part is about twice as long as in other shooters of today (it of course depends on how fast you run through - many levels have lots of "eastereggs" to offer you won't get to see if you just run by...

You just need to be aware that it's for like 80-90% a "classic" shooter. Don't expect cover systems or blinking arrows telling you where to go (not that this would be difficult to figure out in DNF although it has its moments where you have to sort out the right way to proceed). Also don't expect any fancy AI. You'll also need a good portion of humor to enjoy this...

For me it's the best shooter I've played in a long while (not that I've played too many though as I don't like what most modern shooters have to offer). I especially like its diversity and level design. You don't just crawl through the levels and just shoot stuff - there's always something interesting or new. I think this is where the game 3DRealms wanted to create in the first place really shines through...