The last s40 phone I used was the 7250 way back in 2002, then got my first s60 (7650). After that I've had a 3660, 6630 and now a 6680 and there's no way I'll ever get a s40 anymore.
The thing I like the most is the multi-tasking so I can send sms's or check my calendar or phonebook while I'm in a call. (I suppose the latest s40 phones still don't support this..?)
The huge amount of available apps and the big screen don't hurt either.
It did take some time to get used to s60. It took me a while to realise that pressing the red end call key leaves the app running in the background so I was always out of memory. And the "special" PC Suite for 7650 was appalling.
Someone once said on another forum that Nokia has never realized that "s40 is a nice phone ui but s60 is a way of life" - so Nokia has never really told their customers about the benefits of s60. Like when a new phone comes out, it's really hard to find out which series it is (ok it's mentioned somewhere in the specs but it always seems to be just a minor detail to Nokia, not a core feature that needs to be promoted.) So I bet there's a lot of people who are totally lost with their first s60.
All those who believe in psychokinesis raise my hand.