Firefox 3.5 isn't that slow, and with all the available add-ons, is probably the most functional. It probably has the best compatibility with websites, as it's optimized to render pages written for MSIE properly, and can render pages that comply to coding standards properly as well.
IMO Safari just looks like Firefox without the add-ons and some buttons in different places, with the speed of WebKit.
Chrome has no add-ons as well, and the interface annoys me a bit sometimes. As for speed, the V8 JavaScript engine makes the browser even faster than Safari. I haven't seen any compatibility problems with any websites so far, but some web pages that felt sluggish using older systems with Firefox work perfectly with Chrome. The good thing is: Chrome is supposed to eventually add support for add-ons. It will be a better browser than Firefox once they get them working IMO.
I haven't tested Opera and Konqueror for a while (at least 2 years ago), but I had problems with some webpages, they lacked add-ons, and the speed compared to Firefox.
In my opinion, Chrome and Firefox come out first