Originally Posted by Maelstrom
If performance is down it might be time to replace. Keep in mind, you sould never change the chain by itself. Cassette and sometimes deraileur cogs too need to go. If you change out one piece of the drive train it will likely perform worse than ever.
That's fools advice.
I've changed many chains before the cassette and or derailleur pullies needed to be changed.
The point of changing and or cleaning the chain is to avoid excessive cassette, derailleur pully and chainring wear.
Consider this:
http://www.yarchive.net/bike/chain_wear.html