Concur is a great tool if you have a dedicated person who can work on the Administration side.
After we signed up, I heard from implementation specialists. Then months went by and I never heard from anyone. About a year later, I needed help on an issue and had no idea who our account manager was or how to reach that person. My colleagues complain that the Mobile app doesn't work properly. My own experience is that it works if you take a picture of your receipt and fill in the needed information on the web version. If you try to do everything on your phone, it is problematic. Adding a custom field is to the platform is difficult. Losing QB Sync doesn't seem like a good idea, but I haven't tried the WebConnector yet.
We initially signed up to be able to integrate QuickBooks; however, Concur is doing away with this. If you want Concur to integrate with QuickBooks Desktop, you will have to switch to Web Connector before March 1, 2016.