It's not at all "useless." Watching videos does not equal actual playtime, no offense.
I used Ness' FS eight times yesterday in FFA against competitive Melee players, and it hit seven of those times (sometimes dealing KOs as well). The one time it didn't hit was when I only had one opponent left, and he was able to successfully dodge. Everyone's right, if there's only one other player, it's not that hard to avoid. In FFA it's pretty good.
Compare Ness' FS to other Final Smashes, and it seems alot better. Lucario's FS never hit me, not once (as Ness, just stay to the side of the stage and airstall with PSI Magnet and PKT2 until it's over... his FS has really bad horizontal range). For PT, the two Links, Zelda, and Sheik, just stay above or below your opponent and it's impossible to get hit. Meta Knight's and Ike's Final Smashes can be avoided by staying out of grab range. Just play an evade game until they use the FS on another player, or in 1v1, until you get a opening to knock it away from them. The ICs' Final Smash, from what I've seen, has limited usage - especially on medium or large stages. Just jump vertically and away from the peak.
Many FSes can be avoided by simply having an awareness of who has the Smash Ball, and then exploiting the weak area of that player's FS. I think Ness' FS is better than these in most circumstances, because there is no specific "avoid" strategy. It passes through floors and covers the stage horizontally most of the time, so the other players just have to *hope* that they don't get knocked into one of the Starstorm comets.
That said, I'm not saying Ness' FS is anywhere near the best. Samus' and Pit's are pretty decent and get in hits most of the time. Kirby's has pretty good range and often results in at least one KO. Sonic, Diddy, Pikachu, and Yoshi (and to a lesser extent Wario and ROB) have excellent Final Smashes in the hands of a skilled player. But above all, Olimar's Final Smash absolutely owns. Every time I saw it used (about five times or so), it KOed all three other players. The initial phase racks up damage, and then the rocket comes down. Olimar's rocket explodes on the ground with ridiculously massive range and knockback. On a small to medium stage, I feel like it's the best FS that I've seen.
[EDIT]
There is an additional advantage to Ness' Final Smash that I didn't mention. When Ness gets the Smash Ball, you can use the FS immediately without any strategic drawback. Many other players have to wait for the right time or angle or whatnot. Essentially, this means that you'll never have someone steal the Smash Ball from you, unlike other characters. It doesn't sound like a big deal maybe, but it's very helpful.