I used to have Moz, NS6 & KM on my machine. I ditched NS6 recently since I only had it for compatibilty testing. KM doesn't mess with Moz config at all so no problems there. Only thing they share in my setup is cache folder. I figured since they're both family they can as well keep their cache together.
Speed-wise performance is about twice that of Mozilla. It's more tangible on slower machines.If your mainly need a browser (for browsing only, not email client, not fancy i-got-this-and-this-and-this browser) then you might give KM a try. If you need hundreds of configuration options (like 20-30 per menu) then Opera is for you.
On the other hand - we're expecting greatly enhanced macro capabilities for new version of KM for all those do-it-yourself type of guys out there