It simply feels nicer to use, from the moment you pull the menu down to reveal your home screen. Its nicer to look at, The phone contact integration is not only cool but useful. Being able to select a persons contact with the swipe of your hand and access their emails, text messages, facebook updates, facebook albums and fickr albums without the need to use a host of other programs is something i would find hard to be without.
The favourite contact widget where you can select how you contact the person is smart. The music player is really nice in looks and features, not to mention the choice of other widgets like weather, twitter, photo album, messages and emails. Also the camera apps are better with touch focus on any part of the image, not to mention pinch and zoom in the browser and photo album.
The thing is people judge what a machine does out of the box, yes you could get similar stuff on the market, but it still wont have that feel about it. The stock google experience simply looks boring, theres now way getting round that and if we want more people to embrace android then nicer looking and more fuctional UI is the way to go.
Sense UI is about connecting to people in various ways and it achieved that goal. Give me sense over stock any day of the week.