According to some tests, the one page checkout process is more relevant than when you have multiple steps (43% vs 35%).
I found it surprising as :
- It was a best practice to split process into steps for having a consistent context at each steps (order > billing > ... > summary)
- Long forms are seen as "repulsive"
But for both of them conversion rate is under 50% - so still a lot of progress to make on such processes.