On the note of cleaning it is far better to use something like white lightning degreaser or give them an overnight submerged in mineral spirits. WD-40 can fail to dissolve some of the lithium grease and not help. I know I sound like a know it all but after taking them apart I can tell you that if you do not clean them well then grit can screw up internals so that they really are broke (unless you are willing to dismantle and modify them, luckily I have heard 8 speed is alot easier to rebuild then 9 speed).

One great way to get free shifters is to bug friends for broken ones and do a cleaning. So if yours start to get weird, clean em. Otherwise if they are good they Are certainly bulletproof except for wear and tear.