I've tryied your class and I must say it works great.
one issue I've found is that if you need to integrate in login form into a website template you get a phpwarning due to a double header set.
so I've only menaged to style the admin form this way:
$sl = new simpleLogin();
$sl->CheckLogin();// this will not display the page if not logged
$sl->ManageUsers(); //Put this line if you wand the admin to see the manager interface
Maybe splitting http header and login form in 2 different object could solve this problem...any idea?