The Singing Detective is possibly the most highly acclaimed television series ever and while I personally rate some other shows higher there's no denying the intelligence, sophistication and passion in Potter's writing, the cast's acting and the overall production. So it's no surprise that the film didn't match the standards set by its predecssor.
Potter wrote the movie script himself, and it struck me as an attempt to simultaneously preserve the highlights from the mini series and make the film appeal to an American audience (shifting the locale across the Atlantic and moving the music up a decade). Pacing wise, being accustomed to the original, the film struck me as rushed. Characters and events didn't have time to unfold themselves. The acting by a high-powered cast was uniformly good though Downey Jr wasn't quite as acerbic as Gambon. Gibson for a change is in "character actor" mode (just to prove he can?) and was just a tad too mannered, but enjoyable nonetheless.