The only place that I have been unable to use it when I wanted to was a Dr.'s office. I bought a $2000 hearing aid and would have liked to have collected points on that, but you can't win them all. I should note that the one major disadvantage to the AMEX is that the interest rate on it is high, so it's realy best for people who pay it off every month. I've had an AMEX of one sort or another for 20 years, so I'm in the habit of doing that anyway.
DH and I each carry a Starwood AMEX, separate accounts. With this card, you get your SPG status set no lower than Preferred +, which means that even if you don't stay often enough to maintain Gold status, you still get 4 pm checkout; worth it right there, if you ask me. However, the really great feature is that DH & I can pool our SPG points, since we are in the same household. As long as you spend one paid night a year at a Starwood property, you never lose any points.
We are leaving at the end of next month on our first redemption stay at the Swan; 5 days for 40K points, no blackout dates. We tend not to use our points for upgrades too often; we like to save them for free stays. It took us 2 years to accumulate enough for 5 nights at the Swan, but it would go much faster if we stayed at Starwoods more often. (Our problem is that most of our business travel involves small towns, where Starwoods are few and far between. We get a lot of ribbing from the Marriott folks when we pay for our rooms with a Starwood co-branded card.)
Another great perk of the Starwood AMEX is the 5K bonus pts. that they give you if you transfer points to an airline program.
Most airlines charge 25K pts. for a coach domestic r/t, so by taking advantage of the transfer, you effectively get the ticket for 20K pts. instead. (That is, assuming that the airline will release a seat for redemption, a rare thing these days.)
I chose not to get the Disney VISA; I have one that has better terms and a nicer cash rebate somewhere that I shop even more frequently. I use the VISA there, but I use the AMEX just about everywhere else.