I will not fly with XL Airways France again.
Cons: XL Airways has a policy that allows passengers to pay for excess baggage up to 24 hours in advance of a flight. The rate is 70€ per extra bag. When I flew with XL, my wife and I were both carrying an extra suitcase containing gear for our respective work. I attempted to pay for these bags 4 times in advance, but XL's online payment system was broken. I documented this with screenshots of my attempts and the error message that their system gave me.
Upon arriving at the airport, the check in staff would not allow me to pay for the pre-airport rate despite having shown them the documentation that I made in advance. Instead of charging 70€ per bag, they charged $140USD per extra bag. The staff (manager included) at JFK was not able to offer us any explanation - "This is the airport rate." XL's customer service has not yet replied to my request for a refund of the difference between the rates.
Having sought out a bargain flight, I really did not mind sitting on an uncomfortable old plane and being served terrible 'french toast' before landing in France (though that did elicit a few laughs), but please don't charge me double for my bags when your website crashed.