in my experienceThat has changed. OmniWeb 4.5 now "uses the WebCore framework from Apple for greatly improved standards support." That's the engine that runs Safari, so OmniWeb is essentially, just another Safari browser, but with one crucial difference. There's a JavaScript Console now, that shows me errors that might be occurring in the page I am developing. The error messages are a bit cryptic, but are still more useful than the errors that Safari gives you (read: none). I would prefer that the error messages were a bit more like Mozilla's though.
In the environment that I work in, we have more than 15% of our users running Safari. That's a pretty high percentage, and definitely passes the "they can't be ignored" ratio (which is, imho, about 8%). Not only can't they be ignored, but functionality can't just gracefully degrade, it has to be on par with IE and Mozilla support. OmniWeb's JavaScript Console makes that easier. If I were the business/marketing wonk down at The Omni Group, I'd be pushing that behind the registration fee. But I'm not, so I'm glad it's not.