I should preface this briefly by saying I've never gone through heroin withdrawal or read the novel.
I really liked the movie. The acting was all-around good, the dialogue (when I could understand what they were saying) was excellent, the narrative unfolded well and the direction/scene composition/location shots were superb. I don't think it matters if it is a realistic depiction of withdrawal, it is certainly an accurate portrayal of depravity. The graphic emphasis on bodily fluids (shit, piss, blood) was very effective in establishing the kind of primitive sort of subsistence lifestyle yielded by addiction.
The direction was outstanding, employing a variety of different styles and techniques. I can't think of a single scene where light, camera placement, location, etc were not done well. The filmmakers used the architecture around Edinburgh and Glasgow expertly for the exterior shots. I particularly liked the way the train wallpaper in Renton's room was designed to look like bricks, giving the impression that he was confined in a cell.
The tone of the film struck an excellent balance between being a humorously sardonic, tongue in cheek appraisal of a bunch of deadbeats while still conveying the essential emptiness of their lives. Consider for instance the way Renton is dragged all the way to the hospital like a sack of beans; he is essentially divorced from his humanity and becomes a vessel from which money is extracted first by the dope dealer and then by the cab driver. There are numerous similar examples.
It has a great soundtrack.
Excellent film.