I believe that Kirk does deserve the promotion in rank. This promotion will also be permanent as it was granted to him not just by Admiral Pike, but also by the Starfleet Council. Also, I don't see why this is such a big issue for some people. After all, didn't Starfleet lose over seven ships, and doesn't Starfleet need to get their best and brightest out of the Academy and into the rest of the ships of the fleet as soon as possible. To me, we got to remember that we are talking about James T. Kirk. Lets not forget that according to TOS, and the first several movies, James T. Kirk proved himself countless times to be the very best Captain that Starfleet has in order to save the day from disaster.
Alright, since this keeps on coming up I'll explain it nice and clear. The reason why rank exists, is so that people can progressively and gradually learn their job. it is not a reward for someone being the most awesome person out there.
You start out small, leading about 40 or so people. You learn the ropes of small unit command. You then do a staff job for a little bit. You then take charge of even larger groups. Then more staff jobs. Now why is this done?
Experience. You can be smart, you can have outstanding leadership potential. That means nothing unless you have experience in both command and staff duties. It is for this reason that the movie makes no sense and stretches credibility. Yes he saved the earth. That's what medals are for. You don't see them making Medal of Honor winners generals. The reason why is because even if you are an outstanding individual, that does not mean you are qualified enough to take a command position.
When they made kirk an instant captain, not only did they stretch credibility by doing so, they also damaged his character a bit. He is no longer an individual that you can relate to, he's this super character with miraculous powers that makes him instantly better than anyone else. Its lame and doesnt do the character justice. It also ruins some good story telling oportunities about a young officer
learning about how to become a leader and what it means to develop over time.