Well I've finally gotten round to making the "Remember Me" checkbox and after extensive testing I think I've worked out all of the bugs.
If it doesn't work for you then please let me know and I'll try and sort it.
If you check it, it will remember all of the details you have just entered regardless of any previous details you had filled in, (so if you leave a field blank that you previously had filled in the cookie will store nothing for that field).
If at any point you submit a comment with the checkbox unchecked, the cookie will be deleted.