I'm giving this a low rating as my experience has generally been poor. My main criticism is that the website allows you to book transport, in my case from my home address to the airport and I received confirmation for this. It was only by chance through contacting them on another matter that i was told my booking had been cancelled. This is not the first time I've been let down by the supplier and it seems Hoppa doesn't necessarily check or communicate with them.
I would always follow up any booking with the supplier directly because the likelihood is they may not have received the booking or if they do they won't necessarily honour it and you'll get a cancellation. This also happened when we were in Italy and expecting a transfer to collect us, at which point we were already at the airport so was a nightmare trying to organise. By all accounts cheap prices are offered and then you get led down.
You will frequently get discount codes offered and these are freely available on the Internet but the customer service is appalling. I couldn't get any refund from them and in the end had to go down the PayPal route to claim back money.
I've only given them two stars because of my experiences, though I did have one occasion when everything worked out fine but I wouldn't take the risk.