It's easy, fast & convenient! I can't think of anything that could make me dislike it at all - even if you're not using their service they are very helpful when needed.
They do charge quite high rates but in my opinion this is worth paying extra as i know what im getting myself into with any booking website/API etc.. If there was one thing missing from flexbokr then surely its an auto payment option so we dont have anymore excuses about why payments haven't gone through yet or were waiting until later (as has happened many times). Try them out!! Their customer support team will be able help answer every question u may possibly need answered regarding how everything works too which makes life more enjoyable working within such software.