With my iPhone I've always hated landscape, but with my iPad I think landscape clearly has the upper hand. But I have to say I use both about the same.
Mainly landscape. In my opinion, it looks asthetically better. Plus, I hate surfing the internet in portrait mode, because it makes the font smaller, and I don't like having to hold my iPad to my face when I read.
For ebook reading though, it depends. I'd say it's about 50/50. Sometimes, I read in portrait, and sometimes, I read in landscape. It depends on the mood, really.
Every promotional shot of the iPad shows it in portrait orientation but I always use mine in landscape, probably to emulate a computer screen. The one downside is that the keys are spaced a little too far apart in landscape.
Landscape, i hate the keyboard in portrait. It also feels better on my lap in landscape. The keyboard feels like a real keyboard. For videos, landscape is a must for me. I hate the massive black bars.