Where Saw uses clever writing and plot twists that are realistic, and yet elusive, Den fails on these points miserably. It's plot is overly contrived and once the film reaches its climax, it soon after meets an anticlimax too soon and after some biblical facts being splashed about the screen, which were poorly investigated, the film ends.
The characters were the strength of the film. The characters were all unique enough to hoard in actors willing to play them, and earned actress Dana J. Ryan a best horror actress award in an independent vote. The problem with the characters is that the film in its tenure took its cast through a frightful experience that taught them nothing and ultimately almost everybody had blood on their hands. The other issue with the characters was that their depth wasn't explored. Facts were shown about the characters. The revelation to the Doctor that his wife's pet name was known to Milton was an unrealistic reaction. It wasn't one of surprise, though nor was it one of inquisition. It was almost spoken atonally and without any meaning.
The greatest flaw of the film was the writing, however the actors surrounding the words given were not nearly strong enough to make a bad script good. They even failed at making a bad script bearable.
If you're interested in seeing films that bear similarities to Saw, see its sequels first and foremost. They're the greatest films of their "new horror" genre. Second to these I'd suggest is House of 9 featuring Dennis Hopper. But do yourself a favour and avoid Den in favour of this.
Even see a Uwe Boll film before this.