Jab cancels are useful against most opponents, but be wary vs Mario (he's the first one to come into my head anyway) because he can easily jab cancel out of YOUR jab cancels... or do a lot of other things too.
Mostly try to use jab cancels into a retreating zair for good spacing, a dsmash for a finisher, or at the very beginning of a stock to rack up a little damage.
Note however that jab cancels can be easily DI'ed out of, even at low percents, so do only one or two in a row at any given time.
Ftilt is good for punishing aerial foes when they air dodge past you, because it the hitbox starts behind Link, then goes over his head and all the way out front. While the lag after the attack isn't too bad, you'll have to act fast to get out of your opponents way if you whiff the ftilt.
It's also a good finisher at higher percents (and comes as quite a surprise too!) and an opponent who is planking or holds onto the edge too long, the tip of ftilt will knock them a solid distance away from the ledge.