There are many great new features in watchOS 4, that on balance it is worth the upgrade. Positives include being able to adjust music more easily during a workout, the excellent new Siri watch face which it almost like an new user interface more akin to Google Wear, but within one watch face. I’ve found my watch is better at realising when it’s no longer in range of my phone should start using WiFi instead. The downsides are the familiar Apple software upgrade problems: it will slow down your watch, so much so that it’s noticeable even when glancing down at your wrist to check the time. Battery life is also slightly diminished in my experience. Not great for any of you who spent ~£500 on a stainless steal model and strap a 18 months ago.
So yes it’s nice to have these new features, but like everything in life, they come at a cost.