Finally got a chance to have a few minutes' play with one in a shop today. Hard to get a good try out when there's a silly metal clamp around the phone, but I have to say it felt much nicer in my (relatively small) hands than the S3 next to it - the width and the curve both helped. As for the screen, my ageing eyes just found it very natural and restful to look at - rather than drawing attention to itself, the screen was good enough that I quickly forgot I was looking at a screen. Very nice.
Since the 64GB version isn't available here yet (Paul at MoDaCo reports that HTC told him it was coming to the UK, but the initial release would just be 32GB) I guess I'll have time to play with the S4 and see whether Google/Motorola do say anything interesting next month. But my feeling was very much that this was a device I could see myself using for the next few years, and I'll be very surprised if the S4 is able to make a better impression on me.