Personally, I enjoyed the Reach campaign. I loved the integrated vehicle-specific levels, and I also really enjoyed the overall scheme of it. One of my favorite parts though, was the way my multiplayer character crosses over into campaign levels. I usually enjoy making my characters as HUGE as possible, so I modeled my character after Jorge, from campaign.
Anyway, overall, the Reach campaign is absolutely epic, and it is my personal favorite Halo campaign to date. I would highly recommend it to any Halo or Bungie fan.
I've included a little clip of some campaign (a little old, but it does the trick), just to give you all a feel for it!