Nothing beats a flat bed for sleep. I went round the world in BusinessClass a couple of weeks ago, and slept on every leg. On the three long hauls that included two 9 hour sectors and one 14 hour sector, I got a total of 20 hours of awesome rack time. In my sleep on my other legs (2 intl. business, 1 domestic first), I was constantly waking up and adjusting my pillow, my legs, and my hips (I'm 6'6").
The only times I half way woke up on the flat beds was in heavy turbulence. What is great about sleeply deeply is that I was too damn tired to have any anxiety about it...I remember thinking to myself - "wow, that's some freaking heavy turbulence (trans-pacific)...zzzzzzzzzz"
As Dan said, sometimes the dimensions aren't ideal. I was worried that I'd not be able to get comfortable on the SQ Biz flat bed, given that whole diagonal arrangement, but once I got my legs straightened, I was good.