From BBC News:
President Ahmadinejad announced the release of the 15 British naval personnel like a card player flinging down his hand to scoop the pool. Iran had good cards and played them well. It made its point about defending its borders, dominated international television with pictures of its prisoners and their “confessions” and, when it perhaps judged that it had got as much as it could expect to out of the confrontation, ended it with a flourish. Iran will project this as a victory (the medals given publicly to the officers who led the operation was an immediate example) against a country still viewed with suspicion in Iran because of its past interventions.
It also put out an indirect warning that any attack on its nuclear plants would be met with vigour. At the same time, the British government can argue that it managed to put enough pressure on Iran to force it to put an end to the confrontation without Britain having to make any formal statement, even of regret, at the incident.
More here.