When I asked a friend of mine's wife what voucher he wanted for his 50th Birthday she said Amazon. Since I knew my friends e-mail address, I thought it was best to use the e-mail option because it might be easier to redeem for him than a printed voucher, but after I sent it, I started reading the terms and conditions which state: "Amazon.co.uk is not responsible if a gift certificate or a gift card is lost, stolen, destroyed or used without permission". I then started to worry that the e-mailed certificate could be sent to a spam folder and then deleted, so I had to phone my friend and tell him to watch out for it and check his spam folder. The other thing to bear in mind is that if you get the e-mail address wrong and send it to someone else, you will probably lose it.
Having said that, they are flexible in that the amount of the certificate goes into the persons account and they can use a credit card to top up for larger purchases or combine gift certificates and if they use less than what you provide, the remainder stays in the persons account for at least a year anyway. I think if I use them again, I will print them rather than e-mail them for security. I wouldn't send them to someone who didn't already have an Amazon account because you are effectively forcing them to open one.