systemd has replaced Upstart as the standard boot and service manager on all Ubuntu flavors except Touch. At the time of the 15.04 release there are no known major problems which prevent booting. The only service which does not currently start is Juju, which will be fixed in a post-release update soon; all other packaged Ubuntu services are expected to work.
Upstart continues to control user sessions.
If your system does not boot after installing or upgrading, please file a bug report and tag it with systemd-boot. Please see /usr/share/doc/systemd/README.Debian about how to debug early boot or shutdown problems.
You can boot with Upstart once by selecting “Advanced options for Ubuntu” in the GRUB boot menu and starting the “Ubuntu, with Linux … (upstart)” entry. To switch back permanently, install the upstart-sysv package (this will remove systemd-sysv and ubuntu-standard).
If you use custom or third-party Upstart jobs, you need to write a corresponding systemd service file or SysV init.d script for it. Please see systemd for Upstart users for a comprehensive guide.