The rendering is very similar to IE 5.x, and Netscape 6. But unlike Netscape 4.x, which takes so long to load you need to pack a lunch, Opera gets these sites visible almost immediately. Pages do look good enough and adhere very closely to W3C standards almost to the letter. Web designers have said that designing a page to look good on Opera requires a ton more effort than even Netscape 6, which is good and bad. Even then, there are some weird rendering problems.
Some pages load up their Ad banners in different places, which throws off the site's look.
Most interesting, some pages don't even finish loading which is as odd as it gets. And the Java applets that would require the Java download crash more often than they work. Some sites do kind of get thrown off by Opera, but it's a lot more tolerable than some of Netscape 6's rendering gaffs. Overall - Opera isn't up there with Internet Explorer, but it's gaining on Netscape with each release. The small download size and speed of the browser make it at least a download for everyone to test out.