Normally I have 0 to 2 programs in the background (e.g. mail, browser, photo viewer, eCoach, SMS, a contact entry, etc...). I would have more, but the amount of memory is far too small for that. I really don't like the feeling when the phone doesn't respond quickly (it should at least show that it's swapping so I don't have to click many times to be sure).

Another reason for not leaving applications running is that far too often they drain battery (e.g. Facebook in a browser window, FCamera, etc). Apple's way is definitely not the right one, but this issue should be fixed somehow.

I like the way tasks are switched (when tweaked so that the camera button short press brings up the dashboard). It looks quite modern and is intuitive (once you have the dashboard in front of you).

Very good that you are asking this - there are many valuable comments here! And a lot of talented people making N900 a better (the best) device, e.g. shortcutd, and Plastic Theme Pack for quicker and even nicer transition effects for context switch!