Many speed differences that can be measured in benchmarks can't really be felt in the day-to-day use.
I just find that having tried IE8 and FF5 and the contemporary Opera at the same time, Opera was the one that stuck. Lots of little things, I guess. Nice that it's doing ok in benchmarks, but that's not really what does it for me. I am not a one for add-ons, really... I get selection anxiety when I see the amount of available widgets, especially knowing that some of them are evil and some of them are malware, so I run Opera as is, and it works.