I have great difficulty in logging in.
I think it's down to the main page being cached - the browser saves the non-logged in page and then shows it all the time, even after I've logged in. The login textbox is difficult to click in (using Opera) - only about the bottom 10 vertical pixels of it seem to count as text. Ditto for the submit button.
Once logged in, I am redirected to xxx.php?, which by dint of being a different url doesn't use the cached version and I see myself logged in. However, if ever I follow a link back to the homepage (eg clicking on the logo at the top left on a results page), I get the original, non-logged-in homepage (from the browser cache)
Unfortunately fixing this is non-trivial and forces you to think about all aspects of caching: client side vs server side, fragment caching etc etc etc. Sorry.