I worked for a medium-sized company (~250 employees) and they adopted Concur (the AmEx travel platform). I could typically find things that were 20% cheaper, but I guess the company felt willing to pay slightly higher expenses in exchange for a simplified management platform.
As a user, it worked fine for routine stuff. But whenever I ran into problems, they took an inordinate amount of time to resolve.
It's hard for me to tell if the things I didn't like about it were because of the platform or the silly rules my company made us follow. But I didn't particularly like it.