Duke Nukem came out in 1996, about in the middle of the flood of sprite based corridor shooters. It was a fair looking game by the standards of the genre, although the quality of the level and character design stood out quite a bit. Dukes one liners were all pretty funny even if they were stolen from older movies like Evil Dead and They Live. The 'raunchy' content (strippers and gratuitous violence) made quite a commotion at the time, but are tame by todays standards (in fact it was overshadowed pretty quickly by intentionally 'shocking' games like the original Grand Theft Auto). It was very popular for awhile, however the arrival of true 3d games like Quake and Half-Life cut it's popularity a bit short.
So, how does it fair today? Well, the gameplay was good at the time; faster paced than Doom, with more interesting weapons and levels. It can't hold a candle to todays games in terms of technical quality, but it has a certain tongue-in-cheek cheesy humor that is hard to replicate in modern games. The levels still play well, and the weapons are more interesting and fun than most that you will find in modern games (which, I've noticed, tend to just give you standard rifle, shotgun, rocket launcher arsenals).
So, I'd say for six bucks and a two minute download, it's probably worth it as long as you are looking to have a fun time with a cheesy action game, not looking for anything serious.