CiviMail has been a component within CiviCRM since early on (v1.2 or so). However the technical hurdles to install and get CiviMail up and running are quite high and hence CiviMail did not get as much attention or user base as CiviCRM and the other components. CiviMail is an integral part of any medium-large size install and is used by a few organizations successfully.
For the average user, a few things have changed in the recent past which I suspect will increase the popularity of CiviMail. Some of these changes are:
We've taken this opportunity to branch a "CiviMail" version from 1.8 so folks can potentially get access to all the above killer changes earlier rather than wait for the 2.0 release. All 1.8+CiviMail changes are being tracked on our issue tracker
. This is a work in progress and usual disclaimers apply :)
If there are features that you think are essential for CiviMail, please start a discussion on our CiviMail forum