He was taken off the programme because of his increasingly mad behaviour I think he hit his wife and did a lot of stupid things (male menopause?) There was talk of replacing him with Ashton Kutchner don't know if they have. If you google Charlie it might tell you.
It gets better as it goes on - the first few episodes with AK were pants. Charlie has been in a new sit-com called Anger Management (based on the film), but I don't know if it's a regular role or if/when it'll be shown here.