I got a Series 1 when launched but I upgraded to Series 2 (SS) three months later. The main reasons for the upgrade were the SS case (looks nicer) and on-board GPS (running without my phone was a game changer for me).
The sapphire screen, water proofing and brighter screen were all bonuses to me.
But I still kept my Series 1 watch (and the almost two years old Series 0). One is gold aluminium and the other space grey. Having different case colours is nice when switching to certain bands. Also allows sleep tracking.