On a competitive level... it doesn't seem to be a lot of competition among the regular playlists (to date); omit Arena and MLG primarily. I feel the gameplay isn't consistant enough to satisfy most of the 'competitive players', in other words the game isn't focused in that respect.
On a casual level... it's great fun. I spent a whole entire day in customs just messing around on forge maps just going mad creating things. And just the other day I spent almost two hours in Co-op finding different places on the map in which we can access, as it turns out; there was a lot to explore. Also the AI in the game is fantastic, your fighting each enemy as an individual, and they way that Bungie crafted the levels; you can approach a situation in many different ways.
So... if not for competitive online matchmaking, why does Reach still bring in close to 100K in players on a good day?