I was still using the S0 until a week ago, and for the basics - notifications, reminders, activity tracking, and telling the time - it was fine. A bit slow at times, but perfectly usable with OS4. I think the performance actually improved with each OS version. Now it has been relegated to gardening duty and sleep tracking, which it does well.
So far, the improvement I have most noticed on the S3 is the much brighter display. It is much easier to read outside on a sunny day, while wearing sunglasses, which I love. The S1 has the same display as the S0 I believe, so for me that would be reason enough to pass on the S1 and look for an S2 or S3 instead.