First reviews of the Apple Watch Series 12 and Ultra 4 highlight key upgrades despite identical exteriors. The S11 chip powers faster Siri AI responses and enables heart rate variability (HRV) measurements up to 24 times more often without battery drain. The Verge's Victoria Song found Siri AI queries noticeably quicker on the Series 12 versus the Ultra 3, with 20-minute charging boosting battery from 20% to 64%.

9to5Mac's Chance Miller confirmed the Ultra 4 achieves 48-hour battery life in real-world testing, including workouts and sleep tracking. A new Readiness app dynamically updates scores 0–10 throughout the day — unlike static morning scores from competitors — and provides underlying data free, whereas Oura Ring charges $7/month for trends. CNET's Vanessa Hand Orellana noted this continuous adjustment lets users see how a midmorning nap affects readiness.

Reviewers require at least seven nights of data for Readiness scores, so most lacked scores after a week. Audio Intelligence features Siri Rewind and Live Recap arrive later via watchOS update. Engadget's Cherlynn Low called the models a "solid upgrade" despite missing launch features.