I'm still not entirely sure why they went for that title. Either way, everyone can relax because it's good.
There's actually a lot more of Porno in it than I expected. The central scam is different and generally of less importance, and the focus is split across the main four characters a lot more evenly than the book (which almost felt like a Sick Boy solo novel at times).
Porno is a lesser book but I'd say this adapted it about as well as the first film did, and where they diverge they often managed to avoid the weaker points and add depth.
The main criticism it's going to get is that it's very much a love letter to both the first film and the books, and it's not subtle about it. It gets a little cute with the winks at times, and towards the end it turns into an orgy of self-referentiality that comes close to breaking the fourth wall, until...Warning: final shot spoiler! (Click to show)
It's a film about itself.
But the characters are still great, the actors are still great, Danny Boyle dials his Danny Boyleness up to 11, and it tickled, repulsed, shocked, scared and moved me at various points. It is what it is with enormous gusto.