The last few weeks I've been using Epiphany
as my primary browser, and basically Epiphany knocks the socks right of Firefox. Epiphany's user interface looks a bit more clean and it's a bit more snappy than Firefox'. Epiphany is mostly GNOME HIG compliant, so it's easier to use, and it fits better in the GNOME desktop environment.
Yet I switched back to Firefox... why you ask...
First and foremost, Epiphany doesn't have a separate Google search box, instead when something is entered into the address bar which isn't an URL, it's sent to Google as a query, but somehow this doesn't always work properly, which can get pretty ennoying.
Another issue in Epiphany is the fact that every tab has it's own close button. Logically speaking this is better, and is actually required by the GNOME HIG, but practically this sucks (at for browsers it does). The main difference with browser tabs and text editor tabs (and the like), is the rate at which tabs are opened and closed. When compared to a text editor, tabs within a browser are probably opened and closed tenfold. For the user this means he has to visually locate the tab, then click the approriate close button on the tab. When considering the single close button (like Firefox has), located at the right, the user has one fixed position to click, which (generally speaking) is much more convenient.
Last, Epiphany still doesn't have del.icio.us
plugin, which I have quickly grown attached to, after trying the Firefox plugin.