These things seem to be insanely rugged, as specified in their datasheets. Watch these demos.

This is BBI Amps from the US, one of those guys who like to build crazy high power CB amps. Love them or hate them, they are entertaining. And some of them build some beautiful looking amps. This guy builds all sorts of solid state- and valve amps. In this video (starting at 21:40) he is showing the ruggedness of his latest amp using the BLF188XR LDMOS FET.

In this video (fun starts at 1:30), the ruggedness of a Freescale MRFE6VP61K25 is demonstrated. It would be interesting to know how much of this can be done before it starts degrading, if ever.