I just upgraded to the series 2 from my series 0.
Main reasons:
- I had a hard time seeing the watch in bright daylight - Series 2 is perfect out in daylight now. Much more brighter.
- My battery was getting under 10% many times by 6pm - Series 2 fixes this.
- Waterproof: I love that I no longer have to take my watch off before I go swimming. The Series 0 might have been fine, but it didn't have the water-lock feature. When I wore it in the rain, water would hit the screen causing all sorts of fake random clicks/swipes putting me in/out of various modes and changing my watch faces. It was super annoying. Now I can just click the water droplet, and that won't happen.
- Siri: Siri seems to actually work on the Series 2 - failed most of the time on the Series 0.
- Future proofing: I bet many things in WatchOS 4 won't work on the old Series 0.
GPS? I don't need that feature.
Speed? It's the same speed as the Series 0 for most everything 'right now'. I'm sure that WatchOS 4 will make the Series 2 faster than the Series 0, as it will take advantage of the dual cores.
Speed wise, I think the S2 processor is significantly faster over the S1 Chip. Mainly with start ups, updates appear faster and applications. I don't think it's drastic, but it certainly is noticeable one the S1.