We've been using Concur for about 2 years. It works fine, and I get reimbursements promptly made to my checking account, provided that my boss promptly approves the expense report :-)
I do NOT book my airfare through Concur, using the corporate AmEx account. I book it on my own, so I can use my own Mileage Plus credit card. I had knee surgery last year, and didn't fly much, so I lost my Premier Gold status with UA. Therefore, I now opt to book flights using my Premier Club card in order to get benefits such as Premier Access, free bags, etc. I'm careful that the flights I select meet the company cost guidelines (price is no more than $200 higher than the lowest fare). So far, all my flights have passed the Concur auditing process, and have been approved.