I hate unless statements.

customer.friend_orders.subtract(self) unless customer.nil?

# the above is equivalent to the below:

customer.friend_orders.subtract(self) if ! customer.nil?

The if statement is patently more understandable to my brain. I tend to stare at the unless statement and sit confused for 2 minutes, like a monkey trying to work a calculator. I try to piece it together in my brain - and it all fails, until I write out the if statement.