Just have received my custom order! The stamps look great! We choose a picture of us and the process was very easy. It just took 5 minutes and the stamps were here within 2 weeks.
I did it because I think it's a nice touch to our invites and it could prevent confusion with junk mail! ;-)
The layouts are limited but anyway...it's the pic that really matters!
P.S. The stamps are permanent "peel off" for domestic use only. If your invites are heavier than the normal limit, you will need to add more stamps depending of the weight. Also, if you are using the custom stamps, it's better to ask the staff at your post office to cancel them manually. Otherwise, a machine will print on your face! ;-)
The pic is in bad quality but the stamps look great!