I hate them, it defies the cascading nature of CSS, and if you don’t use them with care you end up in a loop of adding more !important.
But I want to know are they bad for performance?
EDIT
From the (fast) replies I can conclude it won’t have a (significant) impact on performance.
But it’s nice to know, even if it’s just as an extra argument for discouraging others ;).
EDIT 2
BoltClock pointed out that if there are 2 !important declarations the specs says it will pick the most specific one.