I use Concur because my crappy corporate employer purchased it. My strong suspicion is anyone who matters makes his or her assistant use concur rather than doing it him/herself.
The product is absolutely awful. Like just stunningly bad.
Most of their assistants just use AmEx travel (or some other travel agency software) -- which has a much higher per-seat license and actually ties in to the back-end reservation systems so you can see all flights, etc. As far as I can tell, Concur sources its data through site scraping in order to achieve the lower per-seat licensing costs (since flight data feeds are very expensive).
The product is absolutely awful. Like just stunningly bad.