I use firefox 3, chrome 2 beta, opera 9.6, ie7, and safari 4 on my atom netbook.
firefox 3 is slow to open, but only because i use about 10 addons with it, and keep about 20 combined tabs open between two windows. I have the toolsbars minimized in size and have a fullscreen button. Once its open and cached into ram, it runs fine. I use firefox tweaknetwork, adblock plus, tabmix, fullerscreen, download helper, foxclock, zoomtoolbar, and a few others. pretty solid, and i like how it remembers authenticated sessions.
ie7 is the standard, have to use it for windows update, and to stream media player internet radio for my local stations. Its the slowest of the 5 and only comes in as a backup.
safari 4 is quite fast. I love the top viewed sites tab. It blacks out the screen and gives it the ipod/iphone interface feel to the browser. I have a similiar addon in ff3 that displays all of my open tabs in a large grid with a dimmed background. safari is fast, neat, and streamlined. It uses multiple processes and a lot of ram like ff3.
chrome / chrome2 beta is fast, and loads quickest of the bunch, like opera. I like that it has the tabs in the title bar, yet it isnt very customizeable. Overall it is the fastest, and i really like how it opens each tab as a new process. It also has a nice home page that organizes bookmarks and frequently visited sites. It has a download tab built in which is nice. It is the fastest of the bunch, but cant block ads like ff3 without forcing the addition of adblock via coding. I really like firefoxs addons. My dell mini 9 has no apostrophe key so excuse my punctuation. chrome is good.
Opera i just got a few days ago, It has a built in torrent downloader which is very neat. It has plenty of customizeable buttons in the toolbar, and overall it has the FULLEST fullscreen ever. this is great for netbooks. Once i was viewing a high def photo fullscreen and dragging it around via mouse gestures and middle click. I was in fullscreen mode, and when i right clicked, the menu didnt have a way to exit fs mode. I couldnt f11 out and for a second i felt like a moron stuck on a fullscreen image. Very nice fullscreen to sum it up. heheh. opera runs quick too, and uses one process for multiple tabs. Very customizeable, takes some learning to get used to mouse gestures. Its about as fast as chrome2 with more options.
Im using mostly chrome for speed and simplicity. Firefox is second for addons and customizations. opera and safari are backups, and ie7 is used only when needed.