Dear Scream,
I've done this, and received the following report from phpmyadmin:
Table Op Msg_type Msg_text
threads.w3t_Users repair info Wrong block with wrong total length starting at 1570136
threads.w3t_Users repair warning Number of rows changed from 631 to 5170
threads.w3t_Users repair status OK
It said "repair status OK", but my pages now have twice as many SQL errors as before the repair. I'm really desperate now.