If you haven't seen it yet, I demand that you go watch it.
It leads perfectly into Halo 3. It is epic and awesome and moving and just awesome. If they had made the Halo movie and it had been like this, it would have surpassed every other movie I love and become my hands down favorite.
And mods, I didn't see this anywhere else on the site yet, but I have a tendency to post these things after there's already a thread, so my bad if it's already there.