Actually I must disagree and say that Identity columns are nice and convinient but can be very bad as well. Why would someone define an identity column on a table that already has another unique key, specially if that column is an int or bigint?

Recently I had living proof that is not a good practice.

I have one database that was developed with the maxim that identities are never enough, and this one table had a natural key (unique, int) that was theonly field used in the application, and had the identity, that was the clustered PK.

I started noticing dead-locks on that table, and the dead-lock was between updates on the PK and the unique key. Since all access to the table was via the UK, when deleting from that table the order of the locks was the reverse of the selects, and it resulted in dead-locks. Getting rid of the PK (and the identity all together) got rid of the problem as well.