Short version? 2001: A Space Odyssey meets Event Horizon.
I loathed Event Horizon, and I didn't particularly like the aspects of that film that Sunshine evokes either. But more later.
Long version? I quite liked most of it, but have some reservations.
The design, lighting and effects of this film are faultless, evoking Kubrick's 2001 quite loudly in places, as does some of the script. If this doesn't get nominated for best visual effects at next year's Oscars, I'll eat my montera.
The characterisation was a bit broad, but at least it was by and large believable. It was also great to see scientists portrayed as something other than social misfits, hermits or genii-on-the-edge. They (well, most of them) make life-or-death decisions with the clarity of judgment, coolness of approach and professionalism one would expect from the scientific crew selected to save the planet. The massive weight of responsibility on this mission (from an unseen Earth) affects almost every crewmember, and this creates is a tension remarkably well teased out through the entirety of the film. Remarkable because for 99% of the movie, you don't see the Earth at all.
There is no single standout performance because there is no clear lead, but I would have to say that Cillian Murphy is the one that left the slightest impression, which is surprising given the amount of screentime he has towards the end of the film. Chris Evans starts out with an unintentionally funny "Ron-Burgundy-on-the-skids" beard and mop, and he is quite unrecognisable until he shaves it off. Nonetheless, he puts in a solid, if typically alpha male, performance.
There were two actors whose performances perhaps wooed me more than the others, despite relatively brief screentime. The first was Cliff Curtis, as the officer with a toddler's quasi-spiritual desire to stare into the sun. The second was Hiroyuki Sanada as the dignified and measured Captain Kaneda, whose first thought is always for the mission, and then for his subordinates. There is a fantastically haunting moment between the two of them, which you will miss if you're not listening closely, just before the Captain parts ways with the rest of the crew.
Many people have started berating/defending the last act online already. This is the Event Horizon bit I was talking about earlier. Personally, I don't like it; it introduces an additional element of stalk-and-slash that lowers the otherwise intellectual tone of the film.
On the one hand, the "new element" is an extension of the spiritual theme introduced earlier in the film. On the other hand, it is not a natural extension of said theme, and it's a little stretched. It feels like an all-too-convenient plot device which reeks of laziness on Alex Garland's part, given how well the film was motoring through crises and deaths before the introduction of the "new element". I can think of a dozen ways for the third act to have followed a similar or identical arc without adding a "new element" so late in the game. Sure, it might have taken a bit of graft to get it to fit, but it can't have been a greater contrivance than what is there now. Furthermore, the "new element" has no part in the resolution, whether spiritually or emotionally; The way the film ends, the "new element" might as well not have been there.
A quick aside, if I may... I am glad to see that once again the British are once again getting to see British films before anyone else. Between Danny Boyle's movies, Children of Men and Edgar Wright's films, I no longer feel short-changed about our tax-dodging cousins getting to watch our films on the same day as we do, while we often have to wait months to watch theirs. The current trend for massive simultaneous rollouts for blockbusters on a global scale has gone some way to redressing the balance, but I am glad that we have something to gloat about over this side of the pond beyond the superior standard of most of our films.