As for paying merchants online (opposed to in an app , that is supposedly coming:
http://www.recode.net/2016/3/23/11587214/apple-pay-coming-to...
The first generation of chip terminals were horribly slow, they took far longer than a swipe. They also wore out really fast - the spring contacts would stop making proper contact with the card leading to failed transactions.
The current generation (fourth I think) is much faster and more reliable. They can read the chip faster than contactless, there's still the PIN entry though.
I'm not sure how it is in the US but here most retailers rent their terminals so get reasonably regular upgrades.
In Canada, we've had contactless terminals everywhere for a few years, so it's very rare to find a place where I can't just tap my Apple Watch or iPhone to pay. There's no need to find a place to explicitly supports Apple Pay, it works the exact same way our credit and debit cards have for years.
PS: What's with the downvotes? No positive experiences with a product are accepted?
In the UK, at least 1% of all transport payments in London use Apple Pay – a pretty huge volume considering. It's also universally accepted wherever there is contactless support (which over here is now becoming ubiquitous).
TfL's network has supported direct use of contactless payment cards for > 6 months now, so Apple Pay (and the Android equivalent) support comes for free.
On-device payments are possible, and those work pretty nicely. I've used it with Uber before. I don't think it's possible to use them with random merchants who don't have iPhone apps, though.
Since the sticker/card independently already hooks up with my devices anyways (sends me a confirmation push to my phone) I don't actually understand why I need NFC on the phone.