Very few problems - perhaps the same or less than I would have experienced with v3.
Very Good: Very noticeable speed increase in page rendering.
Good: Top Sites - I'm starting to like it. It needs a bit better way to customize what's there, other than pinning ones which show up automatically.
Good: Coverflow History. Has some good possibilities. It needs some speed improvement in rendering older sites.
Not good at all (IMO): tabs on top of window. Very non-standard interface. I've set it back to the v3 method (tabs under tool/bookmark bar). There are some apps make this modification, but I just used the Terminal command.
So-so: The "clock" loading icon. I can see the idea of combining it with the reload and cancel load, but there's no progress (unless you have the status on at bottom of window, but that's not graphical). There's a way to set it back to the blue progress bar over the URL, but I've left it in the v4 style to see how I like it.