So after using the '89 all through Splatfest (except one match at the end with the Octobrush after finally hitting Roller Coaster King), I think the new-weapon-hype has worn off, and I gotta say...
I'm still really feeling it.
It's basically perfect for Splatfest/Turf War (are there any differences between those, outside of lighting), it has decent range/speed/spread for combat while still being good for inking, this alongside smart Sprinkler placement (and some Special Charge Ups) gave me very frequent Inkstrikes (which are always welcome), which let me cover quite a bit of the map practically on my own since I wasn't helpless against enemies and didn't take a while to cover turf like I usually am.
Also, Sprinklers are just awesome for this weapon. Throwing one into enemy territory is like throwing a Hadouken; it doesn't do that much damage, but the other team has to do something about it, or I get to cover their area and charge up for the next Inkstrike (which refills your ink so you can toss out another Sprinkler, which charges up your next Inkstrike, creating a feedback loop of inky destruction unless you get splatted). Also, you can throw it on the crane in the top area on Saltspray and it'll ink most of that area in time (again, giving you even more Inkstrikes; this is the best thing about the Sprinkler by far, it gives you quite a bit of Special just by existing for a bit if you put it in a good spot).
And this from here on is just pure conjecture right now, but the '89's probably great for Splat Zones too, since it has Sprinklers to easily cap nearby Zones and Inkstrike to contest those farther away (or just team-support, dropping an inknado on a spot is a fairly effective way to say GTFO in my experience) while still being decent for combat thanks to its fairly good rapid-fire. Really, its only weaknesses are its relatively low power per shot and its lack of answers to combat outside its max range, which can be bad but don't cripple it.
Its usefulness should be reduced in Tower Control, since Sprinkler/Inkstrike is much more suited to turf-grabbing than tower-capturing, but it isn't useless. It still has its pretty-solid rapid-fire, and Inkstrikes are easy to aim at the tower's location for GTFO or ahead of it to move or destroy future threats to your ride; Sprinklers are a bit more questionable, though I suppose you could throw one on the spire in the tower's center to spread you're team's ink as it travels.
So, yeah. The N-Zap '89 is awesome, you should give it a shot if you haven't.