It can be very useful indeed, but situational.
Parseltongue is one of those character traits that can be very useful if you design your character around it and make sure you get into situations where you're useful. It is not useful if you want a character that just hits everything in melee with brute force without having to think about tactics or strategy.
Here are some examples for uses of Parseltongue other than what you and anotherguest have mentioned.
Firstly, in Antoine de Saint-Exupéry, the Little Prince, the snake gives the Little Prince some advice, and lets him return home when he eventually wants to leave. Although in this book, the Prince uses ordinary human language to speak to the snake, it is clear that he understands the replies of the snake but the pilot narrator does not, so understanding snakes is a special ability in this universe.
Secondly, in Rudyard Kipling, The Jungle Book and The Second Jungle Book, Mowgli can speak to snakes, which has helped him many times. Quoting The Jungle Book, chapter "Kaa's Hunting" first:
Then he [Baloo] turned aside to tell Bagheera how he had begged the Master Words from Hathi the Wild Elephant, who knows all about these things, and how Hathi had taken Mowgli down to a pool to get the Snake Word from a water-snake, because Baloo could not pronounce it, and how Mowgli was now reasonably safe against all accidents in the jungle, because neither snake, bird, nor beast would hurt him.
Baloo can speak in general, but cannot pronounce the words of water-snakes, whereas but Mowgli can do both. Thus being able to speak to snakes is clearly a special ability in the Jungle Book. This ability is useful because now snakes would not hurt Mowgli. Indeed, in the same chapter, Mowgli uses the Master Word in the ruins, and it protects him from the venomous cobras living there.
Mowgli had previously befriended Kaa, a large python snake, who rescuse him from the monkeys in the very same chapter. Kaa's friendship becomes useful again in The Second Jungle Book chapter "Red Dog", for fighting the dholes. Being able to understand snakes' speech was probably necessary for befriending Kaa. Finally, speaking to cobras comes up in the chapter "The King's Ankus" too, though it's debatable how useful it is for Mowgli.
Of all the abilities I list here, this one seems to be the most close to the Parseltongue ability as used in the Harry Potter books, since makes Mowgli safer from snakes attacking him, and let him befriend a snake Kaa who helped him, similarly to how the snake Nagini helped the Dark Lord.
Update: you might want to also check the S.W.I.T.C.H. series by Ali Sparkes, featured in the question Children's graphic novel series, schoolboy twin protagonists retrieve formula that lets them transform to insects . In this series, the protagonits boys (and sometimes other humans) can transform to various insects and reptiles, including snakes, and while transformed, can talk to other animals of the same species. This means they can talk to snakes when transformed to snakes. At least once they can also talk to rats as well while they're transformed to an insect.
The ability clearly seems useful for the boys. Most often, it helps them communicate with each other, which lets them coordinate to save each other from danger. Talking to their sister the one time she was transformed also helped, as did talking to the rats. The boys learn a lot from it, and sometimes it moves the plot ahead too. On the other hand, the ability to understand other ants (ones that weren't transformed humans) was also dangerous to them.
Update. Consider now Arthur Conan Doyle's story "The Adventure of the Speckled Band" from The Adventures of Sherlock Holmes. In this story, a man attempts to assassinate a sleeping woman in a locked bedroom by sending a venomous swamp adder, "the deadliest snake in India", through a small hole in the wall. Had the plan succeeced, the truth about the murder would never have been found out, for the snake would return to his owner through the hole and never be found.
This man did not speak Parseltongue. Although he could handle the snake enough for this attempt, due to Sherlock Holmes's timely intervention, the angry snake returned through the wall and has killed the man. Now imagine what weapon such a venomous snake could become in the hands of someone who does speak Parseltongue and can control snakes! For one, this man would probably not be killed by the snake, though he might still go into prison. But if you can control a snake, you could probably use it as a weapon in a more clever way than just sending it through a hole, and assassinate people without anyone being able to trace the snake back to you.
Update. Harry Potter and the Methods of Rationality by Eliezer Yudkowsky, a Harry Potter fan fiction, has Harry, Salazar Slytherin, and one other character who spoke Parseltongue. This came very useful to at least the third character, although I won't spoil the exact details here because they're revealed close the end of the book.
