to use the newsequentialid function. I've never bothered with it unless
my tables had several million records in them.
Page splits can still be avoided though if your indexes having enough
padding in them and are rebuilt occasionally.
As far as the security concern of someone guessing another GUID in your
database-- you would probably want to have extra checks in place rather
than just loading whatever record came in the URL anyway. (For
instance, letting someone only view the profile of the logged in user,
even if they passed another user's id in the URL)