Agreed, I make almost all my purchases with my Amex card (mostly because of unlimited 2% cash back on everything). It's usually smaller independent restaurants that don't take American Express. It's something that I never really understood, because American Express in my experience charges lower transaction fees than Visa/Mastercard. When I had an Amex merchant account they charged a flat $7/month fee with no per-transaction fees, though our volume wasn't very high.

However, you have to setup the American Express account separately, which probably has a lot to do with why many mom and pop type restaurants don't take it. But now that a lot of new restaurants seem to be using processors like Square, this is a complete non-issue.

Yes, Square is definitely helping solve some of the issues for places like that.

I shop at farmers markets quite a bit, and a lot of the merchants there are using Square, and it is awesome =)

That's not my experience. I have to use an Amex card for work to make IT purchases all the time and I'd say a good 25 to 30% of the places I try to use Amex at don't except it.

I am a normal consumer, I have never used American Express for business purposes. 25 - 30% seems really high though, considering American Express used to be targeted mainly towards business users...

I've found several SaaS sites that only take Visa and Mastercard, but not Discover or American Express.