DF is worth having for nostalgia value. As Pedro says, it's the game that started the Jedi Knight saga.
It was very cool for its time, and for being basically a Doom-clone. And five pounds is fairly cheap (though you may find it for cheaper if you look among "used" games; usually just the CDs and jewel boxes, no packaging or manual or anything).
OTOH, that was a while ago, and to a modern gamer, DF's graphics are boxy and cartoonish at best, and the enemies are basically cardboard cutouts that stop looking good at 20 paces, and the bosses are impossible to beat without cheating. There's no third-person option, no MP, and precious little editability (though TACC does have some fan-made DF levels).
Ultimately, it's your choice. If you're a hard-core JK fan & collector, by all means go for it.
"The Beasts know much that we do not." -Ancient Jedi proverb