After reading Aza Raskin's post about Firefox moving its tabs down the side of the window, I decided to give it a go in Opera. It turns out to be very useful when you have a widescreen monitor. I usually end up with several dozen tabs open at once and it's much easier to be able to put them down the side in an area which is, for most websites at least, dead space. On the rare occasions I do find myself on a website which requires more than the 1470px horizontal space this gives me, I can just tap f4 and get my full 1650px back. As the window sidepanel also groups by window and lists all tabs open across all windows, I can keep them ordered thematically, too.
This arrangement definitely doesn't work, however, when you have a small screen. When I tried this on my netbook, I had to choose between losing half of my screen to the tab list or only being able to read the beginning of each page title, even if I only had one tab open.