I like it because it's our most useful spacing tool. 95% of it you know, but I hope I can provide that extra 5%. Massive tl;dr. I'll just color useful parts
It's range is just about as long as marth's fair, so not many characters can out-range it. This means that not many characters at all can directly challenge Ike's nair. Unlike Marth's fair, Ike's nair goes just about all the way around him, making Ike pretty safe against people trying to juggle you if you're fully retreating away.
If you face away from your opponent and nair, if you time it right by landing while the hitbox is behind you while still retreating, he can't punish it. Just whipping out nair while your opponent is chasing you is effective on most of the cast.
Kimchi told me how you can full hop + nair onto ledges, which is pretty useful. Also, on irregular surfaces, fast-falling an nair the correct way could give you a slide, which is pretty funny. Useful on brinstar, for example.
I made a post a while back about juggling with nair. The hitbox hitbox starts above you, which is a useful place to start, so you can follow opponents through their entire dodge animation. Nair can be used as a frame trap in order to get past opponent's air dodges. It works wonders on characters like snake and wario, mixing up between nair and jab. Since many characters can't directly challenge nair, they try to get around it, but they can't really air dodge, either. Ike can also fast fall the nair in order to jab or grab or shield opponents who are still in the air.
The best thing about nair is that it pays off to get a hit with it. On heavies, jab after nair is effective, but doesn't work quite so well on light characters with quick aerials. Back air after nair is pretty cool when it happens, but be wary of danger if you miss, so know when to retreat your bair or not. When I'm feeling daring, I dash + charge an upsmash and wait for the air dodge. Against characters who don't have quick and fast aerials to get away, might as well do another nair.
The attack I do most after nair is just uptilt, since I'm not usually in danger if I miss. On stages like battlefield, it could lead to you getting loads of damage from them landing on platforms. I may follow up with an aerial pivot grab, or upsmash.
The good thing about getting characters caught in nair, is that if they lose or just don't have their second jump, It's an easy pivot grab-->backthrow+dash attack.
In combintion with Ike's other moves such as jab nair provides us with plenty of mixups as well as a decent close quarters defense.