i'm surprised; it actually works pretty fast as well (since it's IMAP and php) we're mainly working out details of how people should login, how to get ssl working correctly (ie if we do webmail.yourdomain.com, we can't obviously buy an ssl cert for each one of your sites, but if webmail.yourdomain.com redirects to webmail.dreamhost.com we could have a cert for that).
we also need to see how scalable squirrelmail proves to be - and how popular the webmail service will be. obviously if people come to depend on the service, and it becomes so popular that the server(s) are way overloaded, then we'll need to either try a different system or use more machines.
if you'd like to give input as to what features are most important to you, this would be helpful (ie speed, security, convenience, reliability). of course we'd like to have all of the above and then some... however in reality, compromises must be made.