I used onward ticket in May. I have never been asked for proof of ticket in 17 years but just said what the heck and gave it a shot.
Website was easy to use. Can "book" any flight itinerary of your choosing for any dates of your choosing. You can also specifiy a time you want the 48hr clock to start on your reservation.
So, for few bucks I believe it is a good insurance policy for that overly inquisitive immigration officer and will probably use the service again if traveling on one way ticket.
Just recalled there was one instance I was asked for onward travel proof. Flight from US with connection in Hong Kong was delayed so missed the connection. Airline rebooked our flight for next day and put us up in hotel for the night. The China Air agent would not re-issue my ticket without onward itinerary. I booked cheapest flight she could offer, to Vietnam I think, for any date she selected and made it clear I would cancel the flight just as soon as she handed me the ticket. But that is only time I ever had a question, was in 2009 as I recall.
But anyway, onward is good to go for peace of mind.