Like all of the Final Fantasy games, Final Fantasy X has an ultimate, or celestial weapon for each character. Tidus' is called the Caladbolg, and it is a pain in the ass to get. I loathe Blitzball, so I personally think Wakka's is the worst to get, but Tidus' is right up there. The Caladbolg is useless until it is powered up. You need the Sun Sigil and the Sun Crest.
The Sun Sigil is the hard one to get. At the far northwest corner of the Calm Lands, Tidus will find a woman on a chocobo. She will lend you a chocobo to ride if you train one for her. Training a chocobo is nothing short of pure hell. Seriously, chocobo training is cruel and unusual. You can start it as soon as you access the Calm Lands. There are several steps, and I recommend breaking it up and doing it at different times to save your sanity.
The final chocobo training challenge is a race against the Chocobo Trainer. As you race her, birds fly at you, stunning you if they hit you. There are balloons along the path which you can collect as well. Every bird that hits you adds 3 seconds to your total time, and the balloons are there to offset it. For every balloon you collect, 3 seconds are shaved from your time.
To acquire the Sun Sigil, you not only have to beat the Chocobo Trainer, but you have to beat her with a time less than 0.00 seconds. I've broken many a Playstation 2 controller and have dented many a wall in previous apartments.
To get a time less than 0.00 seconds, you'll have to get roughly 12 balloons and get hit by no birds. You can alternatively get 13 balloons and get hit by one bird, or get 14 balloons and get hit by two birds, etc. This is as annoying as it sounds. It's going to take you some patience.
The only tip I have is to make sure you get the five balloons at the start of the race. You race down a slope before turning completely around and running back in the direction you came. There are roughly 5-6 balloons on that slope, and the Chocobo Trainer will try to get them all. I ignore the first balloon, because the Chocobo Trainer is going to get it every time, and I go for the second balloon, toward the right. Getting this will push you in front of the Chocobo Trainer, and she will push you into the other balloons on the slope. Once you get off the slope and turn around, try to get as many balloons as you can without getting hit by birds. Ignore the Chocobo Trainer's position, as you can beat her even if she arrives at the end first.
Completing this torturous task will reward you with the Sun Sigil.
The Caladbolg can be picked up in the northern end of the Calm Lands after you beat the Chocobo Trainer. You'll need the Celestial Mirror to open the chest. The Sun Crest can be picked up after battling Yunalesca at Zanarkand Dome. Make sure you get the Sun Crest right after the battle, because once you get the airship, the Dark Aeon Bahamut will be guarding it, and it'll be much more of a pain to get.
Once you have all three items, take them to Macalania Woods, up the rainbow path, to the flower you charged the Celestial Mirror with. There you'll be able to charge up the Caladbolg. The Caladbolg is nothing short of amazing once you power it up. It has counter abilities, an ability which allows you to do more than 9999 damage, and a triple overdrive ability which puts Tidus is overdrive more, allowing him to execute his more powerful attacks more often. Is it worth getting? It depends on how much you're leveled, and how much time you want to put in the game. Personally, once I complete all the side quests, I am over-leveled for the last area of the game, and I end up wrecking face with or without the Caladbolg. It is cool, though, and getting all the Celestial Weapons will get you a trophy and the PS3 and Vita versions of the game.