I've used Omniweb since its NextStep days. Last year though I switched to using mostly Safari. The big reason was the startup time. Omniweb remembers your last pages loaded, even when it crashes, and reloads them upon startup. A great feature but time consuming. I intend to switch back to OW because on my recently purchased MBP I found it started reasonably fast again.
It has several features I haven't seen on other browsers. Workspaces allows me to to have certain associated pages grouped. It can, at a site level, set preferences such as browser id, image loading, font & color & style and use a custom style sheet for the site. One feature I've used in the past is to edit the HTML of a page I'm viewing to clean up a bad layout, or in pre-CSS days, change the color of text from the background or vis-versa. Such as light green text on dark green background.