Apple has finally released iOS 9, its latest mobile operating system — and while it's not a complete overhaul, there are a number of key changes to the operating system.

But just how does the new operating system stack up against its predecessors? Since its release, the operating system has been reviewed over and over again. Here's what experts in the tech world think of iOS 9.

Raves

• Christina Warren, Mashable: "Apple Maps and Siri – two things that used to be punchlines – are now good. In the case of Siri, the assistant is so useful, I've begun using it more than I ever have."
• Andrew Cunningham, Ars Technica: "iOS 9 takes that foundation and builds on top of it without radically altering things, much in the same way that iOS 6 built on top of the advancements in iOS 4 and iOS 5."
• Romain Dillet, TechCrunch: "Many of iOS 9's improvements help you do things faster, from Proactive Assistance to the back button that appears when you click on a link."
• Dan Seifert, The Verge: "On the surface, iOS 9 looks like a very incremental update. That's not really a bad thing — it's more stable and efficient than iOS 8 was and remains a reliable and easy-to-grasp platform that everyone from beginners to power users can take advantage of."
• Brandon Chester, AnandTech: "There are definitely some big changes such as the addition of Apple News and Transit in Apple Maps, but these are again just strengthening the core services of iOS rather than adding incredible new abilities and features."
• Mark Sullivan, VentureBeat: "One of the marquee improvements in iOS 9 is Siri's progress in the direction of Google Now. The personal assistant has become better at learning about you and anticipating searches or functions based on your habits and location, as well as the time of day."

Rants

• Christina Warren, Mashable: "I'm still unsure how big Apple News will be — and in truth, of all the parts of iOS 9, it feels the least-finished. The product itself works well but not all news publishers have optimized their articles to fully take advantage of the format."
• Andrew Cunningham, Ars Technica: "The worst thing we can say about the new release is that its biggest, best new contributions – the things that make the iPad feel more like its own device and less like a big iPhone – are only available to a sliver of existing devices."
• Dan Seifert, The Verge: "Google has been pushing predictive intelligence for years, Microsoft has launched multiple generations of the Surface tablet / laptop hybrid, and Facebook is also working directly with publishers to accomplish many of the same goals as Apple News. And iOS 9 is still far from showing returns on any of these forward-looking bets."
• Brandon Chester, AnandTech: "Apple still only provides you with 5 GB of free storage, and if you want to back up your iOS device to iCloud it counts against that storage. I think Apple is in a position where they could offer free users 15 or 25 GB of storage for free, and not have their device backups count against it."

Conclusions

There are certainly a few negative things that could be said about iOS 9, however, almost all the reviews suggest that there's no reason not to upgrade. Even the negative things were already negative things in iOS 8 — so upgrading to iOS 9 is a no-brainer for those wanting to take advantage of the new features that the operating system has to offer.