Let me start by saying the Aranet4 has completely changed how I think about indoor air quality. As someone who's battled headaches and fatigue for years without knowing why, this little device revealed my home's CO2 levels were through the roof (literally). The moment I saw those numbers spike above 1000ppm, everything clicked.

The e-ink display is genius - crisp, easy to read, and so energy efficient I haven't even thought about batteries since setup. I keep mine on the kitchen counter where its subtle design blends in but remains visible. That color-coded CO2 reading? Lifesaver. Green means breathe easy, red means 'open a window NOW'.

Where this device really shines is accuracy. After comparing with cheaper monitors (yes, I went full nerd with side-by-side testing), the Aranet4's NDIR sensor consistently delivered more reliable readings. The calibration process mentioned in other reviews? Did it once outdoors and now I trust its numbers implicitly.

Practical perks:
- Battery life that actually lasts years (not months)
- Travel-friendly size (I've taken mine to offices, hotels, even a cross-country flight)
- Simple app that shows trends (though I wish it stored more than 7 days of data)

The not-so-perfect:
- Price tag will make you gulp
- Celsius-only temp display (come on, give us Fahrenheit folks an option!)
- App could use some modern touches like dark mode

After three months of use, here's my verdict: If you're serious about air quality - whether for health reasons, productivity, or just curiosity - this is worth every penny. That moment when you realize your 'afternoon slump' coincides with CO2 spikes? Priceless. Just be prepared - you'll become that person who insists on opening windows during winter meetings.