Just tested it with a small module;

UserInfo currentuser;

protected void Page_Load(object sender, EventArgs e)
{
lblMessage.Text = "Update city";

currentuser = UserController.GetUserById(PortalId, UserInfo.UserID);

if (currentuser != null)
{
lblMessage.Text += "
For user: " + currentuser.Username;
if (currentuser.Profile.City != "")
{
lblMessage.Text += "
Current: " + currentuser.Profile.City;
}
else
{
lblMessage.Text += "
No city field found... ";
}

String newvalue= DateTime.Now.ToShortTimeString();
currentuser.Profile.City = newvalue;

UserController.UpdateUser(PortalId, currentuser);
lblMessage.Text += "
New value: " + newvalue;
}
else
{
lblMessage.Text += "
No user " ;
}
}

And it might work the first time, but after trying it again, the value stays the same.
So is this a bug in DNN 8 ?

Really really?