Do task killers actually work?
Task Killers do generally "work" in that they kill tasks.

They don't "work" in that most apps that you'd want to kill due to running constantly will simply be restarted by the OS, and Android already has appropriate task management built in.  A task manager will mostly just drain your battery and waste your time.

See also: How can I stop applications and services from running? and the question eldarerathis links in the comments.