I'm no expert either but you may want to think about how these usernames are going to be made up. just letters? alphanumeric? any other characters to be allowed? using wildcards can sometimes be dangerous as anything will be accepted.
anyway. I think I have a better solution.
Code:
RewriteEngine On
RewriteCond %{REQUEST_FILENAME} !-f
RewriteCond %{REQUEST_FILENAME} !-d
RewriteRule ^([A-za-z0-9]+)/?$ members.php?id=$1
it checks that teh request filename doesn't exist.
These two lines check that the request filename doesn't exist.
!-f - not a file
!-d - not a directory