I'm not entirely sure.
On the one hand, it's fast. Like, really fast. I zoomed across any inkable surface when I was using this thing. And its attack is pretty fast too. Also, the Sprinkler is a nice, complementary sub-weapon for the Inkbrush, since it covers an area independent of you. And the Inkstrike is a pretty nice Special, at least in my eyes.
On the other hand, it leaves a very thin ink trail (roughly as large as a Seeker's trail), so it doesn't cover much ground, even if it does cover it quickly; furthermore, it feels like it has very little range when swung (that could partially be my poor aim, but even when I held it upward it didn't look like it was going that far). Also, it doesn't feel like it deals much damage per hit (does it deal any damage when you're dragging it on the ground?), so it won't get quick splats except maybe on already-weakened enemies. Also, the Sprinkler takes a lot of Ink per use, so you will probably have to recharge after throwing one (or before, at least in my experience). Finally, the Inkstrike doesn't really feel like it belongs with this weapon (just a personal preference, I think).
I could see it being useful to get around really quickly and to get your teammates to places quickly (I've thrown Seekers out in front of me and followed behind them to replenish my ink, go forward quickly and maybe attack at the same time) and it could be a great harassment tool since it's so fast. Again, the Sprinkler feels like a great addition to the Inkbrush, which can cover quite a bit of space while you're off doing something else. Ninja Squid feels like a must if you're using this thing, so you can sneak up on people, and Swim Speed Up too, so you can run away quickly. Maybe both Ink Savers and/or Ink Recovery Up would be useful, if only so you don't end up running out at a bad time and can recharge quickly when you need to.
Maybe it would be better if it were stronger (maybe they'll make a Strong Inkbrush at some point), or had more range on its swings (please let a Long Inkbrush happen at some point), or if it had the Bubbler or Inkzooka or Killer Wail or Burst Bomb Rush or just something other than Inkstrike as its Special (please, please, PLEASE let a Custom Inkbrush or Inkbrush Deco happen down the line).
Right now, the Inkbrush honestly feels like a less extreme version of the Rapid-Blaster-to-Blaster comparison to me. It's faster (much faster, unlike the Rapid Blaster, which felt slightly faster, but not faster enough, but that's just my opinion), but I don't think the loss of power (and in this case, range) is worth it.
At least, that's how I feel.