My friend had this as a birthday present and we spent all yesterday checking out the maps on custom games. The only thing I would say is that textures in the distance don't get drawn until you are pretty close to them - I saw a lot of rocks that looked like lumps of plasticine for example. Other than that I thought that it was a massive improvement on Halo 3. I'm so glad that they changed the look (and feel, for that matter) of the game. It looks considerably less 'cartoony' and much more realistic. One thing I was really glad of was that now it acually feels like you're firing and hitting someone when using the assault rifle. In Halo 3 you would get the visual feedback (seeing the armor flicker from being hit) but it would almost feel like nothing was happen on your end.
EDIT: The addition of the jetpacks, holograms, etc works very well. It adds another level of fun to the game which I wouldn't have expected. If someone had told me (I didn't follow much of the news surrounding the game) that these things were being added I probably would have said that it would ruin the game...