I may actually watch this one again, and I almost never watch anything again.
We love Martin Freeman, right? But in this one, he's not so likable -- except he's still sympathetic because he's him. So it's hard not to root for him. The very end... nah, I won't spoil it. :-p
Warren Clarke doesn't look like a movie star, but he certainly fit this role perfectly. He was entirely believable in every way.
The continual shifting between narrators was a bit confusing once in a while, and I sometimes found myself trying to recall what situation a given narrator had going on. The device is clever, but is it good movie-making? Alternatively, is it good in principle but stretched too far with 4? You tell me. It's the main reason I might watch this again: to "get" all the angles.
Finally, I just have to say that Nina Sosanya with those dreads is unbelievably enchanting. What a fox!