We have just put a store live with Magento 1.9.1 CE & have Paypal Express enabled.
All seems to work ok on Mac, ios and Android but on windows using IE11you click on Paypal Express & it take you through to the Paypal login that's ok, click continue & that take you back to the paypal express review page but when you click "Place Order" you just get a spinning wheel on the web page as if the order is going through but nothing happens.
Works ok with Firefox on PC.
Went to the backend & turned off Paypal Express Review but it still goes through the the page on all browsers & platforms.
After speaking to Paypal who hadn't come across this problem before, they suggested that a similar problem was caused by not having all of the software updates for Windows 8.1.
As this was a new laptop supplied to my son becausehe has severe dyslexia by a specialist company I assumed that it had been fully updated, WRONG.
After several hours & several hundred updates later the problem is now fixed but be aware that if people are not able to order this is possibly the cause.