It all depends on whether the added features and updated features as well as functionality of GS8 justifies in your mind the value of the upgrade.
Is it a quantum leap forward from a GS3 - GS5? No, but there is enough to justify the upgrade even considering the substantial price increase.
From design perspective, you are getting a phablet sized screen on dual curved device that is not much larger than GS7 without the physical on-screen home button and capacitive keys flanking it making the GS7 look dated.
In addition, you can switch the back and open apps buttons. The screen is brighter to the point that, at 30%, I can use my GS8 in the car. Multi-tasking, web browsing, and YouTube videos benefit from the increased screen real estate.
Battery life is better due to more efficient chipset and display. You can also customize settings from a power management perspective not possible in the GS7. While edge panels may not be new to some, the Data Management panel has been useful.
Camera has a new but similarly spec'd sensor of the GS7. FFC has been upgraded from the GS7. Has similar fast charging and data storage speeds as the GS7. Dust and water intrusion has been updated. BT 5.0 allows more distance from paired devices as well as pairing multiple devices to the GS8. Base storage has been increased to 64GB.
I have a GS8 playlist on my YouTube channel, TheJCP006, that has reviews that include comparison to the GS7 as well as tips and tricks to further fine tune your GS8 should you decide to pull the trigger.