My elder brother received his X10 today and I spent an hour or two with it. Here is what I have to say.
1- Regarding its touch sensitivity, I see no difference between iPhone and X10. It's highly responsive. Back in mind, I had a thought that SE may not be able to bring that responsiveness but it proved me wrong. Whether you are scrolling through contacts, menu, Timescape, etc, etc... It's brilliant.
2- SE's UI is quite useful and in my opinion, I like it more than HTC's Sense UI. Because the way it brings all the stuff at one place, Sense UI doesn't do that. Animations are really cool.
3- There are only three screens. HTC Desire has 7 or 9. But it's not a something very important. Three screens are enough to accommodate all your short-cut icons.
4- Absence of Multi-touch is hardly noticeable.
5- Couldn't understand one thing. In wallpapers, one can flick through them by sweeping finger but this feature is missing in photos. You sweep your finger but instead of moving right or left, new pic just loads.
6- 1GHz processor's performance is marvelous.
7- Camera result is very good and features are satisfactory as well.
8- Some people wrote about keyboard problem but I didn't experience it.
IMO, this phone is really worth buying. One just can't ignore it just because of absence of multi-touch.