I had the Series 2 and hated it. I got it because I wanted to be able to listen to podcasts/music while having a basic GPS (wasn't training seriously for anything, no intervals, hills, fartleks, etc.). The GPS was good enough, but it sucked for music/podcasts, and had annoying issues like: 1) in the winter, with long sleeves, it would stop recording because my sleeve covered the back of the watch a little; 2) it wouldn't pair bluetooth headphones consistently; 3) it was hard to press the buttons sometimes, causing issues with stopping, starting, etc; 4) syncing podcasts was basically impossible w/o a long convoluted process; 5) heart rate monitor, which I don't really care about, barely worked.
The whole experience sucked.
I was ready to trade-in for a Garmin but the Series 4 announcement intrigued me. The main sell for me was finally getting a watch with LTE so I could be reached without carrying a phone. Essential for me with two kids at daycare, one who has a minute but non-zero chance any moment of needing to go to hospital (hemophilia). The bigger display, faster processor, better OS with podcast syncing (on Overcast and the native app), improved HRM, etc. seemed like it was worth trying.
So far, I'm really happy with it. The bluetooth headphones pair instantly, never any issues. Overcast and native podcast app both sync podcasts, and that issue is solved. Strava and RunKeeper apps both work better with the faster CPU. The bigger 44mm screen makes the button substantially larger, which makes stop/start better. It also feels more responsive. The HRM doesn't glitch. And the speed of the CPU just generally makes it a much better experience.
For me, the benefits of the watch (running without a phone, seeing messages, mail, quick text responses, etc.) and the adequacy of the GPS/running apps make it a no-brainer for someone who seldom does workouts and doesn't really care about small differences in distance logged. FWIW, I ran a 5k this weekend and it was within 0.015 miles accuraacy. Good enough for me.
If you're routinely doing interval-type work with lots of starting and stopping, it may not be the best fit. For a mostly-just-going-for-runs runner, who enjoys podcasts -- especially if you need to be reachable -- it's great. Really happy. And I say that as someone who cursed for 2 months at how shitty the Apple Watch 2 was.