An excellent zoom lens,

+ Sharp at 600mm with F/6.3, at F/8 the difference is negligible.
+ Zoom and focus rings feel very nice.
+ Excellent stabilization.
+ Firmware 1.2 improves the lens on all levels.
focus accuracy and speed.
+ The ratio between price and image quality

- Weight
- With the lens hood, although very well made, I can't say I liked how it attaches for transport.

Overall with the Panasonic S1R body that I have it reaches 3.4 kg Quite heavy but not prohibitive.

Very good lens, I use it on Sony a7iii, the lens is a gem!
It has two significant drawbacks:
A. Weight - It is quite heavy and combined with the camera, it will tire you out.
B. Filter cost - It has a large diameter and the filters are very expensive.
Apart from these two, for wildlife photography, it is the top value for money