First impression: Wow!
I didn’t have the time to have a deep look at them but I am really impressed. I am not impressed by their new interface (even if I find IE’s new interface quite nice), their new features (like embedded RSS reader, etc.) Nah, I am impressed by what make a Web browser a good one: rendering speed and standards complying.
My primary Web browser is Opera. Why? Speed, standards complying, multi-platforms, best tabs handling, the faster DOM manipulation, etc.
The first thing that stroke me when I both used IE and FireFox was their increase in speed. Web sites that were extensively modifying DOM document, on-the-fly, were much more faster (the speed difference between IE 6 and IE 7 with Talk Digger is awesome).
Users have a much better experience using these browsers now. Not for their new goodies or their new user interface features. Users have a much better experience using these browsers because they are much faster, they are much smoother so they are much natural.
Kevin Berton wrote about this dilemma sooner this week: more features or better performance (so better features)? In that case, better performance, so better features was the answer to the question (in my humble opinion).