Apple Pay was launched in 2014 with the iPhone 6 series and is currently supported by the iPhone 6, 6 Plus, 6s, 6s Plus, and the Apple Watch. The iPad Pro, Air 2, mini 3, and mini 4 can also use Apple Pay but within apps only.
Apple had a target of physical 1.5 million locations for the 2015, and it managed to surpass this milestone by a lot. Today Apple Pay is available at more than 2 million locations globally and is continuing its expansion. That's still far from a wide-spread service, but its future sure looks more promising now.
The next wave of supported merchants is going to include Chick-Fil-A, Crate & Barrel, and Au Bon Pain. Popular food chains like KFC, Dominos, Chiliís, and Cinnabon will be adding Apple Pay later on.
Apple Pay will also expand its reach to China later this year. Currently, it is available in four markets - USA, Hong Kong, Spain, and Singapore.
Finally, Apple Pay got an enormous increase in in-app payments, a feature not widely advertised but often used nevertheless.
You're correct - in 4 markets but AFAIK = US, UK, Australia, Canada (latter 2 Amex only) - definitely not in Singapore but can use at some terminals
Give it some time, it will become mainstream.