I think its worth it. The only other Halo game I've owned was Combat Evolved, but I really enjoyed Reach. I thought the campaign was good, really evoked a sense of helplessness at times. One of my friends wondered "how can Reach fall with such a badass team on your side?" I think the campaign addresses the story fairly well but there were some parts that seemed unnecessary. All the new features they've added, armor abilities and weapons are a really nice touch. The new updates to Firefight are amazing, new game types and customisation. And I haven't even hit the multiplayer.
10 - Supply Depot (1)
13 - Barracks
15 - Marine (1)
16 - Orbital Command + Supply Depot (2)
16 @100% Marine (1) - Command Center (2)
21 - Bunker (1) + Barracks (2) (3)