It seems that Marriott Platinum members have received the worst part of the deal between United and Marriott. Marriott Platinum only gives you United Premier Silver Status.. while everyone at United apparently gets free Marriott Gold. Here's what I've experienced so far:
1) We know that there is no United lounge access for Marriott Platinum members. (Ironically, I found out about this deal in a Marriott Concierge Lounge, when a United Premier Gold member, who was not even a Marriott Silver, told me about her "free" Marriott Gold upgrade.)
2) Because you're not a "United" Premier Silver but a "Marriott reciprocal" Premier Silver (what?) - be wary of disrespect and even denial of status:
a) At the airport gate (SFO-JFK. T3): While waiting in line for an upgrade, several _non-premier_ standbys were upgraded. There were still upgrades available, but apparently these were not accessible by "Marriott reciprocal" Premier Silver.
b) When calling in, be wary of agents not being aware of this deal: When I flew in early September (SFO-YVR), I called in to United, explained that I was a Marriott Platinum Elite, but they were not aware of the reciprocal agreement, so could not process my request for processing my premier status change. I ended up having to pay the regular change fee.
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Personal commentary - I'm realizing that it was a bad investment to put all my eggs in on Marriott status, and it may make more sense for me to go for United.
As an independent consultant, I pay for most of my travel expenses out of pocket. Frequently staying in cities like SFO and NYC, I typically pay around $400 a night base (~$30,000/year) - staying with the Marriott's even when there are better rates around. Marriott Lounge access is about the only part of Platinum status that I use -- during popular conferences, it's the best way to get readymade hot coffee without having to wait in a long line at the Starbucks downstairs. Half the time I never receive my Platinum checkin gift (and it's often forgotten, too), but I've never been able to claim the $100 guarantee. The manager would always talk me out of it -- and there was this one time, they (randomly) sent me nuts at 11 pm as a way to make up for it -- the unexpected disturbance aside, I'm allergic to nuts.
With the same expenditure, I could easily gain a much higher United status, where I'd get to qualify for Marriott Gold "for free." And, I wouldn't be discriminated for being merely a "non-United Marriott reciprocal Premier Silver."