Easyjet

I have only taken EasyJet. On EasyJet you'll pay extra if you want to select a seat, and you'll pay extra to check a bag. Paying the bag-check fee ahead of time (I did it when I purchased my tickets) will be a lot cheaper than paying it at the airport.

Also check the size and weight limits for carry-on bags. I just take a small tote on board, so I didn't check those limits, but they may be lower than what you'll encounter in the US.

I don't know whether Air France's policies differ from EasyJet's in these matters.

EasyJet is fine -- just be sure you pay attention to their rules.

easyJet is my preferred budget airline in Europe. I've always found the service reliable, but as the others have said it's important to read the Terms & Conditions carefully so that you know what to expect. I've found that it's a much nicer experience if one busy some of the optional features such as Speedy Boarding or Flexi Fares. As I recall that also allows a slightly more generous baggage allowance.