Is email address a bad candidate for primary key?

Is email address a bad candidate for primary when compared to auto incrementing numbers. Our web application needs the email address to be unique in the system. So, I thought of using email address as primary key. But, my friend suggests that string comparison will be slower to integer comparison.
Is it a valid reason to not use email ids as primary key?

String comparison is slower than int comparison. However, this does not matter if you simply retrieve a user from the database using the e-mail address. It does matter if you have complex queries with multiple joins.

If you store information about users in multiple tables, the foreign keys to the users table will be the e-mail address. That means that you store the e-mail address multiple times.