Just started testing CentOS 7
I've been playing with a CentOS 7 VM at home and for the desktop, adding in the MATE environment via EPEL actually does provide a look and feel not too far away from GNOME 2. It's certainly the best migration route for CentOS 6 GNOME desktop users.
From a sysadmin point of view, there are a fair number of changes - getting used to systemctl instead of init.d scripts/chkconfig takes a bit of time though (luckily, the service command works the same way on 6 and 7, although it's usually just a systemctl alias in 7).
CentOS 7 certainly boots more quickly than 6 thanks to systemd, but I think my gripe is mainly with GRUB 2 to be honest. It introduces a level of complexity to the config that isn't really needed. Gone are the days of just editing grub.conf by hand sadly, which was simple and very obvious.