My own personal experience agrees with the need for Advanced Task Killer. When I leave my nexus one alone for a few days, it inevitably starts to slow down (sometime pausing for as much as 10 seconds totally locked up). I'll open adk, and see that I have 40M of free memory. I kill everything, and the phone is instantly snappy and quick again.

It doesn't need doing as often as on my old G1, but it needs it. I don't know if that is because I am running cyanogen mod (5.0.8, which is based on 2.1) or what, but you can pry adk out of my cold dead hands. Maybe I have some buggy apps. But adk makes my phone usable.

Android doesn't seem as smart as it supposed to be, but I still like it.