The first time I ran the Tineco across the bedroom rug, the little ring on the handle turned red, the motor revved up, and the readout said it was finding more dust than it had any business finding on a rug I'd vacuumed two days earlier. That sensor (Tineco calls it iLoop) is the feature that distinguishes the S15 from every other cordless in this price bracket, and it is the reason I'd actually recommend the thing.
What the sensor does in practice is two things. It tells you, visibly, when a section of floor is dirty enough to deserve another pass. And it adjusts the motor power up and down so you're not wasting battery blasting full-power across a clean stretch of hardwood. The combination means I get noticeably more useful runtime out of one charge than the 40-minute rated number would suggest, because most of the floor doesn't need max suction and the vacuum figures that out on its own.
On carpet this is a genuinely good vacuum. The 120 AW sealed suction is in the same ballpark as the Dyson V11 it ties in independent tests, and the rugs in this house look better after a Tineco pass than they did after the Shark cordless it replaced. The anti-tangle brushroll does what Tineco claims; in two months of long-haired household use I haven't pulled a single wrap of hair off the bar. The five-stage HEPA seal is the kind of thing you appreciate if you have allergies and ignore if you don't.
Hard floors are where the trade-off shows up. The brushroll and the front gate are tuned for fine debris on carpet, which means anything chunky (a piece of dry cereal, a chunk of dog food, a small Lego) gets pushed around instead of inhaled. Cereal can actually clog the dustbin intake, which I learned by feeding it a few flakes on purpose. On a clean kitchen floor this matters not at all. On a kitchen floor with kids it matters every morning.
The battery is the other honest weak point. Forty minutes in auto mode is enough for one floor of a small home and not quite enough for one floor of a medium one. The Pet Ex variant ships with a second battery and effectively doubles your runtime, which is the version I'd buy if I were buying again.
The rest is small. The LED display shows mode, battery, blockages, and tangle alerts on a single ring on the handle, which is a clean piece of design. It stands on its own charging base, which not all cordless vacuums do. The brand is less recognizable than Dyson or Shark, which only matters when you're trying to give one as a gift. At its price it does enough things right that I would buy it again, even knowing about the cereal.
Verdict: Buy this if your home is mostly carpet and you want smart features without paying Dyson money. If you have kids who knock over cereal, get a V15 Detect instead.