The iPhone 4S is up for a great start: after one million preorders in 24 hours, reviewers gave it the thumbs up ahead of general availability on Friday. The improved camera, faster processor and Siri voice-enabled personal assistant in iOS 5 are the most talked-about iPhone 4S features among reviewers.

Macworld’s Jason Snell says the S in the iPhone 4S could very well stand for “sure thing.” “All those people who’ve been hanging on to their iPhone 3G or iPhone 3GS, the wait is over: It’s time to upgrade without any hesitation whatsoever. The iPhone 4S has speed, a great [8-megapixel] camera, some cool voice-recognition features, and the same beautiful industrial design that was introduced in the iPhone 4.”

Writing for the Wall Street Journal, Wall Mossberg seemed equally impressed with the new phone: “The iPhone 4S is one of Apple's less dramatic updates, but, when combined with the Siri, iOS 5 and iCloud features, it presents an attractive new offering to smartphone users. Some may be content to skip the new hardware and just enjoy the software and cloud features with older models. But those buying the phone will likely be happy with it.”

For the New York Times, David Pogue writes: “Overall, Apple has done an excellent job [with the iPhone 4S].” A good chunk of his review is dedicated to Siri, the voice-enabled personal assistant exclusive to the 4S, which he praised as “crazy good, transformative, category-redefining speech recognition.”

Stephen Fry wrote in his review for the UK’s Guardian that “Siri, the high-quality and ultra-fast camera, 30 fps 1080p HD video, globally available voice recognition and the introduction of two antennae (the phone seamlessly switches between whichever is getting the strongest signal) are features that make the 4S irresistible; what is more, the unchanged form means that a whole new range of covers and accessories won't be required.”

Brian X Chen points out in Wired’s review “the iPhone 4S looks exactly the same as its predecessor — but who cares? If it was shaped even slightly differently or came in a new color, people would still go nuts over the stuff that’s more important anyway: the insides. And both inside and out, this is a magnificent smartphone.”

At The Verge, Joshua Topolsky says the iPhone 4S reminded him what makes Apple products special: “It’s not specs, services, or apps. […] Is this the best phone ever made? That’s debatable. But I can tell you this: the iPhone 4S is pretty damn cool.”

Daring Fireball’s John Gruber confirms in his review “the iPhone 4 and 4S both have 512 MB of RAM.” Compared to the previous model, “the iPhone 4S has all the best features of the iPhone 4 — same look, same feel, same Retina Display — and adds several significant improvements. The one and only disappointment I have with the iPhone 4S is that the shutdown spinner animation is still low-res. That’s pretty low on the list of nits to pick.”

For TechCrunch, MG Siegler concludes in his review: “Apple focused on the other thing they do best: refining already great products to make them better. The iPhone 4 was a great product. The best smartphone ever made. Now it cedes that title to the iPhone 4S.”