OK...it may be bias, Trainspotting was one of my favorite movies, and one of the better, if frustrating, books I experienced in High School.
And, I really want to avoid any hints towards the plot as....it might kill a lot of the humor for those that haven't seen it. So excuse the vagueness, but after watching it, you might see where its necessary from an entertainment not a twist standpoint.
So...the bad: It is NOT "Porno" which breaks the heart because, well, "Porno" was absolutely hysterical, right down to why it had its title.
That being said, the draw is the cast. Trainspotting was one of those "how the hell did they make this book into a coherent movie" movies and in many aspects "Porno" would have been easier, but would certainly still fit into that mold. So it's likely better that T2 didn't follow the book.
Anyway, the joy is just seeing the crew back together again, the chemistry is still there, it's totally an ensemble cast and that was really the principal draw to the first film as well.
I don't want to give away the plot, but the cast just works so well together, they know their characters and live them. And the characters are delightful to watch.
I sort of feel that they could have thrown any script at them, and so long as the cast and characters stayed the same...the film would have been thoroughly entertaining.
It doesn't hit as hard as the first film, but everyone has aged and it's still....brilliant.
Really, you're watching it for the cast and characters, and it pays off in sheer entertainment.