I've used Opera and Netscape.
I liked Opera for all the good reasons.... its FAST, WAY FAST, two more words "tabbed browsing" (by default) it has LOTS of skins, it kills popups dead. but,

its java implimentations sucked so bad, I kept finding myself using IE to goto some sites so they'd work. Zoom in/out not just font zoom, but real zoom, resizes the pictures too.
So I tried Netscape. almost as fast tho waiting for this pig to load is a bear sometimes. Netscap kills popups just as dead. it has skin support, but there are only 2 skins (probably more, but I never really took the time to search, it just wasn't that important), java works almost as good as IE but I do need to use IE once in a while. it has tabbed browsing, but you need to ctrl click to open in new tab vice new window.
I tried Mozilla to, very same experiance as netscape (huh could be the fact they run on the same engine), but IMHO it lacked the polished feel that netscape had.
generally both have their respective advantages, but don't throw away IE, its compatable with 100% of the sites out there not just most of them, keep it in reserve for when your alternate browser fails you.