The DOM in IE 5.5 is actually pretty stable. You can do a lot by sticking to the W3C DOM, though the IE5.5 extensions are convenient. I know this because I (currently) build intranet web pages for a living; mostly with PHP but for client-side stuff, JavaScript is the only option. We currently target IE 5.5, but we'd like better cross-browser compatibility. In our sights next is IE 5.1/Mac, then IE 5.0/Win32, then NetScape 6 (probably 6.2). Opera is further down the list than it should be because they're actually behind everyone else with their DOM implementation. And Netscape 4 had already been ruled out.
Wade.