I'd have to say Handcent is also my #1 recommendation. I also encourage them to customize it to make it their own. You can put 3 droids on a table and none of them will look even remotely the same. I love that...
So, for me, it's a control issue -- I want a task-killer for the control it gives me, not because it's a required thing to kill off all the background tasks that're going on. So, with that in mind, that's what I tell folks -- if you like the total control that comes with a task killer, get one. If you don't want that level of control, the phone will run just fine without one.
The other apps I tell them to get are the BarCode Scanner, WiFi Analyzer, MotoTorch LED, SleepTimer, SportsTap, Robo Defense, and Sudoku Free.
Mableton, Georgia, USA