Well, I'm a Platinum, and I've flown thirteen domestic segments thus far this year. My "home" airport is SFO. I upgraded on twelve of those, all in advance. (I had to call back several times for a few of those, though.) This is my first year as a Platinum, and I've definitely had more luck since attaining it. Last year, I missed the upgrade on the SFO-EWR Thursday and Friday red eyes fairly often. Now, I've confirmed it in advance every time.
-Adam