Wow. If anything, the 550 looks beefier than my 665. My 665 had rain funneled into the back of it for days upon days, with no ill effects -- and it's got an exposed speaker on the back of the unit that collects rain (and mud, etc).
My friend had a 550 that also did not fail due to water, although I believe his was pretty well protected behind a windshield (mine was too, but the CalSci duct funneled water right into my strombone-mounted 665).
Sounds like ya lucked out! Where's the GPS mounted? Got much water protection from your 'screen?