[__david__]

They do not work on Windows XP (unless the user is using Firefox or Chrome), as the StartCom certificate is not part of the XP trusted root certs. There is an update available from Microsoft to add some new root certs but as far as I can tell it is a manual only update, which means it's very unlikely to be installed by XP users.

We tried to switch our site to https only recently, and had to backtrack because of this (and because we are too cheap/stubborn to buy an SSL cert from someone who is in the XP trusted root certs).

[mike-cardwell]

StartSSL is wide spread enough that anyone who doesn't have their root installed is probably used to clicking their way through SSL warnings anyway.