Trainspotting is highly engaging and riveting. The gross scenes aren't as bad as some reviewers make it out to be (though I do have a high tolerance level). In some cases, there is an added amount of surrealism that makes such a scene work, and that's the main strength of this movie.
The movie is about five childhood friends, four of whom are (or end up becoming) heroin addicts. The fifth, Begbie (Robert Carlyle), while not an addict, is a bit of a psychopath. The movie takes us through their battle with addiction. The depiction of the heroin addiction is effective because of the aforementioned surrealism, which is highly thought-provoking.
Mark Renton (Ewan McGregor) is the main focus of this story, which mainly deals with his handling of heroin addiction. In the movie, he is the only person who seems to have any depth in his character. It is as if the other characters don't possess the "something special" that he has, even though they might be stronger in other respects. I am not sure if this is intentional, but it fits in really well with the theme of the story. The other characters include Sick Boy (Jonny Lee Miller), Spud (Ewen Bremmer), and Tommy (Kevin McKidd).
There are really humourous scenes, enhanced all the more by the Scottish accents. There is a lot of symbolism, both with regards of the heroin addiction, and with regards to the current state of (Scottish) society. There's even a bit of commentary with regards to AIDS. That might perhaps be the movie's major flaw---it addresses too many issues through its use of clever symbolism, from neglect of children to the nature of Scotland's position in the global economy.
Ultimately though, Trainspotting is a fairly typical movie with regards to its anti-establishment theme. It is nothing more than a collection of interesting cinematic ideas stringed together. Each one is worthy of exploration in its own merit, and each one has enough momentum to keep the viewer watching intently, but there is no complete deep message here. Perhaps that in itself is a statement of sorts.