Like many above have said AmEx is pretty useless in Europe. I used to have a AmEx Platinum card before but most stores didn't accept it so it happen too often that I had to pick another store (usually with higher prices) to buy the same item just so I could use to use my AmEx. This is why I later got a Platinum VISA card from my Swedish Bank instead.
An american friend of mine that frequently is in Sweden that owns both VISA and AmEx black card usually encounters problems with his card when paying with them.
1. Because they are made out of metal, when using the chip in the machine that 98% of the stores in Sweden use. It usually doesn't work and show ERROR because it's probably the metal interfering with the chip reader, this leads to awkward situations and 2-3 times until it works.
2. When partying at nightclubs and he uses the card at the bar, the staff wants him (and me when I had the platinum) to open a tab (unusual in Sweden) so that he doesn't have to use the card every single time he visits the bar. This leads to him getting stalked by the security so he doesn't run away with an unpaid tab, which is very uncomfortable for him and his friends (me). They want him to open a tab because they probably don't want to pay the fee to AmEx every time he uses the card.
But the girls dig his card tho. So he has that going for him which is nice.