Have had Diamond status about a year now, have gotten three suite upgrades:
Hilton Sukhumvit Bangkok - one suite upgrade in four stays (January 2014), the other times I was "only" upgraded to Executive Floor King. The suite wasn't much nicer than the Executive Floor King, just a little bigger with sitting area. Wouldn't pay extra for it.
Hilton Guam - upgraded to Premier Paradise Suite (November 2013), great suite, huge room, highly recommended
Hilton Auckland - upgraded to Bow Suite (February 2014), large, great views, highly recommended
All of my suite upgrades have occurred during one night stays, I wonder if that increases the chances.