I have just ordered a Danish "Rejsekort". It may correspond to London's Oyster Card, but covers much of Denmark. There has been critique of the technical system and when I did the registration for the Rejsekort I was underwhelmed by the website.
I saw two spelling/language errors in the interface. One should think that a company that records the Danish personal ID numbers coupled with complete mobility patterns of what probably will become much of the Danish population would be more thorough in spell checking. The first page of the registration gave me a strange time out. Why on earth should the first page timeout? A session should timeout if you are in the middle of a transaction, - not if you are just beginning the transaction. You need to enter a username. This username should be between 7 and 15 characters. Why on earth 7 and 15? Why not 2 and 30? It seems unnecessary restrictive. From the subpages of (one of the) websites there is no link back to a main page. If you press on the image at the top nothing happens. If you cut the path information on the self service website so you get to https://selvbetjening.rejsekort.dk/ then you get to a page that says "Under Construction". HTTPS page of rejsekort.dk does not work. The port is closed. Each time I press "Self service" I get a popup that says "protect you personal data" that needs to be closed. For some of the subpages your need to press two buttons: First the subpage (and then nothing seems to happen) and after that the "see more" button. When you enter data that is not compliant with the restrictive constraints setup in the system the error message does not indicate well where the error is. This is a problem as there are around 10 fields you need to enter. If you enter a username that is already taken it changes the username without any clear indication of the change.
Update: For those who read Danish may take a look on the scary Rejsekort story of Poul-Henning Kamp published yesterday: Rejsekortet: Ingen sanity checks.
Update 2012-09-02: Oh no, yet another embarassment for Rejsekort: The password to their service cannot be longer than 15 characters!