Pros
You get to meet amazing colleagues. Staff travel can be great, depending on how you use it. New joiners' allowance (currently 1200€ in my base) helps new employees on paying the training course. It's very unlikely that you will spend the night outside of your accommodation, great for those who decide to settle in.
Cons
You have to pay for your uniforms and for your training course (organized by another society called crewlink, actual cost 2000€ if paid all at once or 2700€ if paid with your future salaries over the next 6 months). You don't always get to work in the place you asked for. Base transfers are possible but unlikely, depending on which base you're currently working. Even though people want to transfer in a specific base, it's very likely that Ryanair will send new cabin crew in that base instead of first transferring the ones that requested it (there's a number of factors that can be relevant for your transfer request to be approved or left in queue, but none of those are openly specified to the employees).