Just a bit curious since I have got one unit as well. How long have you used your unit before they fail? Mine seems ok so far but I use it very occasionally since I still stick on my old iCN750 unit. I have used iCN750 for some 5 years with no hard problem.
The GPS units are like any computer based item, if treated correctly, they should last years.
I have a Garmin Streetpilot 3 as well, (7 years old) that sits in the spare wheel in the wifes car, used rarely, but never gives a problem.