The Twilight Examiner just posted a great, in-depth article examining the reasons the newest cast members were chosen.
the results have varied, [but] most Twilight fans are pleased with the choices, it seems. A few have said that they think perhaps some of the casting was good, but that the role selection was off (for instance, by stating that Jamie Campbell-Bower would have made a nice Alec).
One thing we can know is that there is a mix, here, in the rationale behind the selections.
Reading through it reminded me where I’ve seen a few of these actors before, and impressed me once again with Summit’s casting.