"The public asked for bigger and Apple delivered not only bigger but better, too," tech news website Slash Gear said in its review.
"A stunning Retina HD display, blisteringly fast A8 processor, and class-leading connectivity make these the best iPhones ever."
Mixed in with details about improvements to cameras and screens was advice that would-be buyers inclined to use smartphones as, well, phones, should opt for an iPhone 6 instead of its bigger-screened brother the iPhone 6 Plus .
"There are a couple of reasons why the Apple iPhone 6 Plus might also be too big for me to purchase," Lauren Goode wrote in a review at Recode.net.
"But I have to admit it: I'm tempted. I really like this phone. And to people who actually prefer huge smartphones: You are going to like this phone, too."
Reviewers said that while the iPhone 6 Plus was a weighty handful, Apple tastefully modified the design to make it comfortable to hold and use.
TechCrunch referred to the iPhone 6 Plus as the "first truly well-designed big smartphone."
New iPhone 6 models boost screen sizes in what some see as the company catching up to a "phablet" trend combining features of smartphones and tablets.
The iPhone 6 has a screen of 4.7 inches and the 6-Plus is 5.5 inches, allowing Apple to adapt to consumers' apparent preference for bigger displays.
"Apple hasn't fumbled," David Pogue of Yahoo Tech news said in a review.
"Its two new iPhones are excellent. Beautiful. State of the art. Worthy heirs to the iPhone throne."
Some reviewers noted that the iPhone 6 Plus is pricier than rival phablets, and that the latest Apple smartphones cost tidy sums if prices are not offset by telecom firms in exchange for locking into service contracts.
Apple on Monday said it had received record pre-orders for its new iPhone models, and that some customers will have to wait for the larger-screen versions of the smartphones.
The California tech giant said more than four million pre-orders were received in the 24 hours after the new devices went on sale last Friday.
"Demand for the new iPhones exceeds the initial pre-order supply and while a significant amount will be delivered to customers beginning on Friday and throughout September, many iPhone pre-orders are scheduled to be delivered in October," Apple said in a statement.
The announcement means Apple may see shortages and long lines at its retail stores when sales begin on Friday.
Sales are set to open at 8 am local time in the US, Australia, Canada, France, Germany, Hong Kong, Japan, Puerto Rico, Singapore and Britain. Some 20 more countries will get the iPhone September 26.
Some of the new iPhones will also be available from carriers and other retailers.
Apple added in a new mobile wallet that will allow consumers to simply tap their phones to pay retailers.