I've been messing with epiphany. (Epiphany version 1.4.6 about is: "A GNOME browser based on Mozilla")
It uses the gecko HTML rendering engine. (As a side note: there are really only a few HTML engines in much use at all: Konqueror's KHTML (Also used by Safari), Microsoft's MSHTML, Mozilla's gecko (Firefox, Mozilla, Epiphany, and others), Opera's HTML renderer (I don't know if it has a specific name), and a couple of others (Amaya, links)).
Problems:
No middle click paste URL, and it goes to URL... something that practically every other browser does in Linux (Konqueror, Firefox, Opera, (KFO) at least and have for years. Select then middle click (pastes previously selected text) is the easiest method of copy/paste in X11.)
The tabs are one size... which doesn't change if you have more tabs open than space in the window, at which time, it goes to even larger sizes. Everything else reduces the size of the tabs, and doesn't use stupid arrows at the ends, until you get huge numbers of tabs open.
Mouse wheel isn't working: it works in everything else (Qt or GTK based) so I must conclude that the problem is Epiphany. Which makes it a pain. Mouse wheels having been one of the best inventions in computing hardware wise, in the past 15 years. (Most everything else has pretty much been just slight improvements.)
Bookmark bar is annoying, it imports Konqueror's bookmarks, which is something firefox doesn't do. (Konqueror can export easily, but it's nice) However, you have to add each link to the bar via right click, show on bookmark bar. Konqueror, and Firefox both use a special folder for it. Attempting to add the bookmark bar from either results in just a pulldown folder, instead of the links appearing on the bar itself.
No drag & drop of URLs (again a feature supported by KFO).
All or none pop-up blocking. Again unlike Konqueror & Firefox, which allow it to be controled either by domain in both, or in Konqueror's case, with a "Smart" mode allowing those brought about by the user's clicking (which "just works").
What it does decently well:
Fonts: Are done nicely and scaled, unlike my particular firefox version (I used the binaries which aren't linked to freetype, which is what provides nice pretty Anti-Aliased fonts for most X11/Linux programs.)
It's decently fast. As to be expected from any modern browser. (Of course, fast has lost it's wow factor since I first tried a CVS version of konqueror 3.2... and it took me a LONG time before I had a page that took longer than 2 seconds between click and full loading of the page. Cox has gotten a bit worse now, so now browser is quite as fast as that anymore.)
Modified fields notification before closing: If you modify a text field such as I were submitting something to a site, it asks you before it will let you close the tab, something Konqueror also has. (Firefox doesn't)
It's gecko, so it is the same engine used in firefox. Aren't that many problems with rendering...
Conclusion:
Epiphany is simply not a good browser for Linux. In GNOME, it has the advantage over Konqueror that Konqueror is Qt/KDE, so the look and feel is different. However, it doesn't really have that advantage over firefox. The bookmark system feels like it's out of Netscape 4. (but I seem to recall that working better for me.) The lack of conventions used by other browsers on Linux, make it very hard to use if you've used one of the others before.
2.5/5 It works, but for anything mormal use if you've used firefox or konqueror before, it will drive you batty.
(As a related thing: Firefox still has issues with cache: I was redoing a webpage, and asked someone to check it in IE, which worked fine, but on Windows, Firefox wasn't working, then I had a couple of people w/firefox/windows check it, and it was fine. So I had him reload it as opposed to using the url... and firefox worked right.)