I've read some horror stories on these rear derailleurs, I have one on an older MTB I just got, after reading the bad I've decided I better change it out, I have a spare Shimano Mountain LX group here and will use that if needed. I guess what I'm asking is are these rear derailleurs really that bad? that they come unglued after so much use? would it be wise to go ahead and swap it out before that happens, if it happens?