I've only had plat for a year (after being Gold) and have seen only 1 benefit - a very nice Welcome gift at the CY in Tokyo on an award stay. Value approximately 1500 Yen (though much appreciated!). Not even worth a $69 meeting, let alone a $350 stay. I haven't had any upgrades, and no lounges (at least no OPEN lounges). It has been a low travel year, but I still thought that *someone* would recognize me differently from Gold, when I got a few upgrades at least.
Given the crappy Canadian credit card deal, I am going to cancel my Marriott Visa card and move my stays to Hilton (which, to be fair, has no Canadian card at all) to maintain my Diamond status.
Dr. PITUK