Dave,
The faders generally go bad because the little string attached to the motor breaks. I personally have never had this occur although I keep a second MCU around in case I need to replace a fader. My buddy has had to do this and it was amazing smooth and simple. There is additional info available on the 'net regarding how to clean/lubricate the faders.
The displays can over time become dim or go out completely but again I've never experienced this. Of course, both items I've mentioned could possibly happen with any controller.
It's important that the firmware version be at least 2.0. There is a revision beyond that (2.1.1 I think) but can be updated via the MIDI port. The earlier versions (1.0 etc.) need an EPROM update to be brought up to speed and the chips are no longer available. Do some research on this as I'm not sure of the revision numbers I've posted.
The overlays are held in place with an adhesive but they come off very easy with no mess left behind. You would definitely want the SONAR overlay.
I had no problem paying $350 as it's a good price in light of the flexibility and workflow increase you will gain with the Mackies.
The biggest problem you will encounter is the desire to buy an XT or two although I operated for a long time with just the MCU. That original MCU (still fully functional) is now my parts unit.
Good luck!