I've had iOS 9 on my phone for five days now and I think the sleeper feature nobody's really talking about is Low Power Mode, not the News app, not the transit directions that only work if you happen to live in about four cities worldwide.
Backstory: iPhone 6, 16GB, which in hindsight was a dumb amount of storage to buy in 2014 and I will die on that hill. The update itself was 1.3GB over the air, which doesn't sound like much until you're sitting at 800MB free because you never deleted any of the seventeen podcast episodes clogging things up. I ended up nuking the Yelp app and two games I hadn't opened since March just to get the download to start. Not really Apple's fault, more a "past me should have been more disciplined" situation, but it's the kind of thing that makes an update feel way more annoying than it needs to be.
Anyway. Once it was actually on the phone, the thing I've used constantly is that new Low Power Mode toggle in Settings > Battery. You get a prompt when you hit 20% asking if you want to turn it on, and it dims things down, kills background refresh, throttles some of the visual stuff, and just stretches out whatever juice is left. I tested it properly on Thursday, left the house at 7am with a full charge and never touched a charger all day, browser tabs open, texting, some Spotify on the train. By 6pm I still had 22% left, which for this phone is basically unheard of. Normally I'm hunting for an outlet in a coffee shop by 4.
It's such a small, obvious idea that I'm a little annoyed it took until version 9 to show up. Android's had battery saver modes for a while at this point. But credit where it's due, Apple's version is dead simple, one tap, no digging through five menus to configure thresholds.
The other stuff in this release is more of a mixed bag. Spotlight search now pulls in more contextual info, which is neat when it works, and mostly just shows me sports scores for teams I don't follow. Slide Over and Split View are iPad-only and I don't own the newer iPad so I can't tell you if they're any good, though everything I've read says Split View specifically (the actual side by side multitasking, not just the slide-over panel) is locked to the iPad Air 2, which feels stingy for something people are going to want on older hardware too. The new font, San Francisco, replacing Helvetica Neue everywhere, is one of those changes you don't notice until someone points it out and then you can't unsee it. It's fine. It's a font.
Battery life claims aside, I haven't noticed the whole phone getting janky the way iOS 8 did on this same device last year, which is honestly the bar at this point. "Doesn't make my two-year-old-feeling hardware worse" is a real feature category for iOS updates now, whether Apple wants to admit that or not.
Also, and this has nothing to do with iOS 9, but I have to say it somewhere: the Volkswagen news that broke this week is completely wild to me from a pure software angle. Forget the emissions and legal side of it for a second, just as a piece of engineering, they apparently wrote code into the engine control unit that could detect when a car was hooked up to an emissions test rig versus actually being driven on a road, and it would quietly switch into a cleaner-but-weaker mode just for the test. That's not a hardware trick, that's a conditional branch somebody wrote on purpose. I keep thinking about the actual moment some engineer typed that logic in and what the code review conversation must have looked like, assuming there was one. Feels like a story that's going to keep unfolding for a while and I don't think anyone fully has their arms around how big it's going to get yet.
Back to phones though. If you're on an iPhone 5 or newer and haven't updated, I'd say do it, mostly for Low Power Mode alone, but budget an evening for clearing storage first if you're anywhere close to full. Don't do what I did and start the update with 800MB free while you're trying to leave for dinner. Learn from my mistake on that one specifically.