The Apple Watch is reaching the wrists of customers for the first time today and the smartwatch's own bespoke App Store has also been launched.
The new wearable - the first Apple has made, and the company's first new product line since the death of Steve Jobs in 2011 - connects to an iPhone but also has new versions of popular apps such as Instagram, Twitter and Flipboard.
The Apple Watch App Store has opened with more than 3,000 apps in it, far more than other wearables have had at launch and already rivalling the Android Wear platform in terms of size.
Apple Watch is not yet available in Ireland and the company says it does not yet know when it will be.
In countries where it is going on sale today, buyers will not be able to walk out of an Apple Store with the new wearable, but that does not mean they cannot get it elsewhere.
The technology giant's first smartwatch is being pushed as a fashion piece as well as a smart device, and while Apple has said it will not have stock in its stores today, a selection of fashion stores will.
Apple retail chief Angela Ahrendts said the selling process would be different for the watch compared to the huge queues normally seen outside the company's stores for other products.
Customers have been able to make appointments to try on the watch for the past two weeks, and instead of walking out of the store with a device, they are having to buy online.
But six high-end fashion stores around the world will have the watch in stock to sell to customers who walk in with an appointment.
The watch, which comes in three styles as well as with an array of changeable strap options, was first announced by Apple chief Tim Cook in September.
Speculation and hype has built since then as more details have gradually emerged about the Cupertino-based firm's first wearable device.
The watch will connect with a user's iPhone and can be used to communicate, as well as track health and fitness statistics including movement and heart rate.
Will Findlater, editor-in-chief of gadget magazine Stuff, said: "In many ways, the Apple Watch is the ultimate gadget. Beautifully made and finished, hugely desirable and capable of a host of clever things, none of which you need, but many of which are nice to have.
"The functionality that could end up most compelling is messaging. The ability to send scrawled drawings, emoticons and even your heartbeat direct to another Apple Watch owner feels personal in a way no other message platform does.
"The one-day battery life and price are the two factors that will put many buyers off. £300 or more is a lot to pay for a first-generation device, especially as Apple will learn a lot about making it better once it's in the market.
"That means the second or third-generation Apple Watch is probably the wiser buy. However, waiting would mean missing out on the fun of living with an entirely new, cutting-edge gadget right now, and for early adopters, that will be too much to bear."
Many are not willing to wait. Since pre-orders began, some analysts have forecast that Apple has sold more than two million watches, and industry experts believe it will prove to be as popular as previous big launches.
Stuart Miles, technology expert and founder of website Pocket-lint.com, said: "I think the watch will follow the same lines of success as the iPad did five years ago. It's an intriguing device that has enough wow factor to pull people in."
But he said the change in buying process - and the online backlog that has already begun - could put some people off.
"The biggest problem Apple looks to be facing is not if people will buy one, it's whether they are going to be happy to wait until they can get one," he said.
The Apple Store is reporting delivery dates of June for Apple Watches ordered now, though the firm said that some customers will get their watch earlier.