My experience flying into Bangkok a few days ago:
I had a one-way ticket into Bangkok. Fearing I wouldn't be allowed on the plane, I purchased a Thailand visa.
After some consultations at the counter, I was allowed on the plane without an onward ticket because of my visa but was told that I might need to purchase a ticket to leave the country when I arrived in Thailand or might not be allowed in. This sounded wrong to me, and I felt very confident (based on this thread and armed with my visa) that I wouldn't be turned away nor would I have to purchase an onward ticket. I told the airport ticketing person that I planned to leave over land, and she told me that I might want to have a detailed itinerary to present on landing.
Out of an abundance of caution, I crafted a detailed itinerary during the long flight showing a land departure into Laos, and I purposefully didn't put my visa number in my landing documents (as explained further, below).
On arrival, I handed my documents to the officer, hoping to just get the 30 day visa exemption so that I could save the use of my visa for the onward portion of my journey - when I crossed back in from a neighboring country. Unfortunately, she noticed my visa. I told her that I wanted to use the visa for later re-entry into the country, but she said that was not allowed. She wouldn't let me have a visa exemption, but she did use my visa and didn't ask about onward travel.
So, the bottom line from my experience: the visa got me on the plane and into the country without onward travel. However, I was not allowed to defer using that visa. So, I'm researching as to whether I will need a ticket out of Thailand to get the visa exemption on my next entry (likely from Cambodia via land or Vietnam via air). I suspect I will need that or another visa.
Anyway, your results may vary, but I thought I would give my recent personal experience.