Beat Halo Reach, ODST and Anniversary on Legendary in a week, co-op two or three months ago. Was awesome. Still haven't beaten 3 on Legendary and am hoping that 343i will see sense an Anniversary-Halo 2. Man can dream.. --- http://www.youtube.com/watchfeature=player_embedded&v=cJbTyFzqZu0
Reach doesn't really add anything to the story. You could easily skip it, Master Chief only has a small cameo in it. Plus I didn't find it that good...and I pretty sure most people agree with me on that.
Not sure about ODST, but that's probably unrelated as well. --- Hair Straightener - Please use above waist.