Is it easy to upgrade CiviCRM using drush?
To answer my own question, it is possible to update CiviCRM with drush but this isn't completely straightforward either.  For the benefit of others I post the steps I took below.
These steps must be executed in the root directory of Drupal:

put the site into maintenance mode: drush vset maintenance_mode 1
clear all caches: drush cc all
take a full backup of site and test that this works on a development server
upgrade CiviCRM: drush cvup --tarfile=&lt;path to tarfile&gt; --backup-dir=&lt;path to backup dir&gt;
upload the latest version of the localisation files and extract to sites/all/modules/civicrm/l10n
delete contents of sites/default/files/civicrm/templates_c/
take the site out of maintenance mode:drush vset maintenance_mode 0
clear all caches: drush cc all

Notes:

I was upgrading from CiviCRM 4.6.6 to 4.6.7.  If you are upgrading from an older version you may find that there are more steps.
The first time I tried to upgrade at step 4 this resulted in an error 'Your database has already been upgraded to CiviCRM 4.6.6' (note that I was trying to upgrade to 4.6.7). Repeating this step resulted in success. This bug appears to have been fixed.
It may be easier to manually delete and replace the civicrm folder as Laryn suggests.
I'm not sure if step 6 is really necessary.