During my deputation trip in October I hired a car from Hertz via the Ryanair website.
It cost £308, which I thought was not bad considering I had almost 11000 miles to do.
However that £308 is not the cost of hiring the car from Hertz. What you actually buy is a "Voucher Credit" of £308 which you can put against the cost of hiring a car. The actual cost of hiring the car ended up being £449. That's almost half as much again.
So be careful.
Apart from that the flight was fine, the cabin crew were friendly, everything else was great.