In my experience, each version has been more stable than the one before. It might be worth waiting until Update 1 if you want to be cautious, but I think V15 is at Upd 2 now.
V15 felt more like a service pack than a major rev, to me. A few new things added, like breakpoints in SCL, detection of PN devices (instead of just central rack like before), and some improvements to the built in 1500 OPC UA server. To me, the big improvements were on the motion side: S120's now officially included in startdrive, and Kinematics support in the T CPUs.