I went from a late 2013 2.6/16 15" to a 2018 15" i9/32. I agree with
@dreubencr that I was expecting more of a difference. I wanted so bad to upgrade when the 2016 came out, but given it benchmarked lower than my 2013, I decided to pass. 2017 was again not much of a bump, so I held off again. When the 6 core and 32Gb showed up, I decided to pull the trigger. I went for the i9, cos mentally I couldn't go from a 2.6 to a 2.6 (I know, I know, different ben, different scales, etc. etc.). Frankly speaking, some days (just a few) I think I should have just gone with the 2.6 and saved $300 and wouldn't have noticed any diff. (Hell, I didn't need the 25hp performance kit on the car either, but I'm happy I got it!) So, whether the i9 is "worth it"... it just depends on how much $300 is worth to you I guess...
Overall though, I'm very happy with the upgrade. I run dual 4k external monitors. While the 2013 didn't officially support that, it did run, but the GPU memory was always running near max, and every couple of weeks it would max out and cause a crash. With 16Gb I would also notice swap in use quite often. The extra system and GPU memory is really nice. On everyday use, it does "feel" faster, but I wouldn't say it is material. Contrary to all the negative press about the butterfly kb, I actually really like it; the 2013 now feels spongy once I got used to the new kb. And the physical form factor... so nice...
I'm also a dev... My build times (maven, gulp) went from 1:05-1:15 down to about 41-45s for clean builds.
I'm also one of the lucky ones that don't seem to be plagued by the BridgeOS kps. (I was experiencing it on wake when I had the monitors connected via Caldigit TS3+ hub, but once I removed the hub, I've not had any more kps, with or without monitors connected)