Not good about the coils & have heard of a few failing but not had it happen to mine yet - touch wood.
My 6000 was a first release & the 11" mono is pretty much the only coil I've used on it (apart from about ~6hrs with the 17"). My mates machine is the same age & he's never had the 11" off it & it's still ok to.
In saying that we both had control box issues that needed return to Minelab for warranty repair so not all rosey for us either.
I think there's been some serious quality control issues with this model & hopefully Minelab have addressed that as it's less than adequate for an expensive detector.
Since my initial problem was repaired & all gear checked by Minelab I can't fault mine though. Very impressed with it overall.