A coworker used them from Southampton to Amsterdam, and said it was fine. She paid extra (not a lot) at the time of booking for a ticket that included the maximum checked baggage as well as flexible ticketing. This came in handy - she was early to the airport (was coming off a cruise ship and so booked a later flight to be sure) and they put her on the earlier flight for no further fee.
"Confusing on carry on luggage and checked luggage and various policies and charges by the different airlines, additional costs for seat selection, etc"
When you are thinking of buying a flight anywhere these days, but especially within Europe, you should do a "dummy booking" - up to the point where you put in your credit card. This way, you can see all the rules and fees. Budget airlines follow their rules strictly, as this is how they make money. So, if there's a possibility your bag is too big or too heavy to meet an airline's carry-on rules, pay in advance to check it. If you don't do this and they stop you at the check in, they'll charge you more than if you had paid in advance. Some will charge you even more if you get past the check in with an "oversize" bag, but they stop you at the gate.
Flybe's rules are very clearly stated as you do a dummy booking (I know, because I helped my coworker book her flight!)