Satisfying because it was to good see Stark get what he deserved from the shotgun of Tom. Satisfying because Luther made some good and right choices, only some of which were in his control.
For example, in the scene on the roof, where Tom with shotgun in hand asks Luther to choose who will die - Alice the evil genius or Mary the pixie - Luther after saying repeatedly he can't choose, says Alice should die. Now I think Luther would be much happier and better with Alice in the long run than Mary, and he knows that, too, but because he's a such a fundamentally decent human being, he can't sentence Mary to death.
And, because the universe is ultimately on Luther's side, despite the terrible counter-examples it constantly dishes out to him, both Alice and Mary survive - Alice because Mary bravely pushes the shotgun away from Alice for moment, and Alice because she uses that moment to put a nail in Tom's neck, a nail she cleverly took from a wall a little earlier.
And this now leaves Luther with another choice to make: with Mary and Alice both alive and kicking, whom will he go with? Alice reliably sets it up beautifully, letting the police think Mary is Alice, escaping, and getting a note to Luther to meet her on the bridge.
Luther first goes to see Mary in the police van - she'll be released soon, when the police realize she's not Alice - and Mary in effect tells Luther to go to Alice. Not only does Luther know Alice is right for him, but Mary does, too.
And this is about as much of a happy ending as well ever see at the end of a season of Luther. But it's more than happy enough for me, because it's the way television should more often be, with the hero making the dangerous, unsafe choice. I'm looking forward to a next season someday, before too long.
See also Luther: Between the Wire and the Shield ... Luther 3.1: Into the Blender ... Luther 3.2: Success ... Luther 3.3: The Perils of Being an Enemy