Ninja

You saw the important part right? "No exp loss." My ninja is only level 30 but so far the only good I've gotten from this 2-hour is not losing exp when I get into trouble. One time, when I had full hp, around level 27, I got almost 130 damage out of this against a smithy in Yuhtunga Jungle. All the uses that I remember prior to that resulted in less than 20 damage. I consider this to be pathetic. If I'm going to self-destruct (and not get exp if I kill the target) I should at least be able to save my team in doing so. Under 20 damage isn't going to cut it. I don't see why this isn't at least 300 damage. Oh well, add it to the already large list of nearly useless 2-hours.

Passive Job Abilities

Why is ninja not a horrible job? Two reasons: 1) Dual wield. 2) Utsusemi.
Either one of these make ninja a cool subjob to have and both together is a sweet deal.
Dual wield allows you to use two one-handed weapons at the same time. They can even be different types, and while you will only have access to the weaponskills of the one in your right hand, you will get skillups on both.
What does more dual wield get you at 25 and 45?? When you have two weapons equipped their delays are added together - so you don't attack twice as fast (not even close). The improved dual wields reduce the delay on the second weapon, so by 45 you're attacking significantly more often than you were at 10. Also, the real reason to wield two weapons isn't attack power/rate anyway. This ability is good because you can get the stat bonuses off a cool weapon twice.
Utsusemi, on the other hand, is unsurpassed as a defensive ability. It adds the effect of "blink" which means the next three non-magic atacks made on your character will not land. Misses are not counted against this either - utsusemi's shadows will last until three hits have been absorbed. Much to my sadness the casting of ninjitsu is rather easy to interrupt so it's hard to get the next utsusemi off if you're still the enemy's main target. There's some trick to "blink-tanking" and the timing of recasting utsusemi but I've not gotten the hang of it yet myself.
To get this spell (100K at AH on most servers) I recommend completing The Sahagin's Stash in Norg.

So as a subjob it's pretty clear that ninja is a juicy choice for jobs such as Ranger and Warrior. Rangers can use two Archer's Knives and have Utsusemi for pulling and for those times their damage draws them hate. Warriors benefit from the extra damage and stats on weapons and can serve as better tanks by blinking even just once at the start of a fight.
Something I'd love to hear a report on is Rdm/nin or Nin/rdm; combining dual wield with one of the en-spells (to get the elemental damage twice).

Ninja as a Main Job
Until very recently I was rather disappointed in this (I use war sj). I got the opportunity to learn blink-tanking and found it made playing ninja quite a bit more interesting. It takes good timing to get the provokes and weapon skills off without messing up the utsusemi casting.
Strangely this puts me into the "ninjas are only good for blink-tanking" camp. I really don't see much else they can contribute meaningfully to a party. Take this with my grain of salt though: it's quite likely I'm far too accustomed to the massive damage I get as a Ranger. Ninjas have a lot more versatility than a pure damage class, making it less obvious where their contributions come from (since they're putting in a little in several areas).
Finally, I have heard rumor that throwing shiruken can yield impressive damage (at equally impressive expense). Maybe I'll test this once I'm filhty rich.

See Also:
Bard | Beastmaster | Black Mage | Blue Mage | Corsair | Dark Knight | Dragoon | Monk | Paladin | Puppetmaster | Ranger | Red Mage | Samurai | Summoner | Thief | Warrior | White Mage