For those that disliked the final act of the film...
I see a lot of people here writing off the final act of the film as pointless thriller material. However, I'd like to offer my interpretation and why I think the finale of the movie was artistically and thematically appropriate.
Spoilers ahead:
The genius of this film becomes clearer if you accept that Pinbacker was never actually on Icarus II-- that he's been dead for years and only appears to the remaining crew members as a delusion. The cinematography tells half the story-- there is a clear shift of tone as the ship goes deeper beyond the "point of no return", and the lighting/focus and stability of a lot of the shots gradually become more unreliable and surreal. The characters aboard this ship are clearly going through a lot of physical and mental trauma and by the time Capa gets back on Icarus II he's feeling some guilt over his decision to change their course towards Icarus I (which winds up accomplishing nothing, and only leads to more deaths).
Notice how whenever Pinbacker is depicted he's shown through blurry, choppy and unstable shots which imply that he does not inhabit the same reality as everyone else. It actually makes sense that Capa would project the image of Pinbacker onto his delusions since there was a parallel in what happened on the other ship (authority figure goes insane and turns on his crew) and that he is also struggling with his own doubt about the fate of the mission/humanity's future.
By the time the end sequence hits, there is a surreal fight scene and reality itself is turned on its head as the camera spins and widens. In the final scene, we see Capa's initial video recording from planet Earth (linking the tone back to where it was at the start, before the characters became unreliable) so we can infer that the "true" ending is that Earth is saved thanks to the crew's sacrifice.
I know a lot of people don't like prolonged 'dream sequences' in films, but here it made sense-- one of the key themes of Sunshine is that life and death; peace and hostility; tranquility and madness can arise from the same conditions. This contrast is beautifully conveyed in the observation room scenes, the sun's exposure has the power to cleanse the mind and spirit (and ultimately sustain life) but also can cause blindness, rage and madness.