Personally I feel the campaign, terminals (unlocked in game but viewed on Halo Waypoint website) and the CGI cut-scenes from campaign & Spartan Ops are a must to any casual or die hard fan. In terms of where the story is going for the next couple of Halo games I place Halo 4 as a campaign must have/play.
I found parts of the story, especially the CGI, exceeding the Bungie games for me. There are also parts of the campaign gameplay that fall short of the Bungie campaign(s) of old. Personally I replayed Halo 2 campaign the most, then Reach's, then CE, then Halo 4, then Halo 3 and finally ODST.
The audio mixing in game suffered a lack of volume controls and the in game effects drown out the music too much. I do love at least 6 songs from the OST1 & OST2 though. The AI has taken hits due to the aging xbox 360 hardware and the high graphical quality of Halo 4, which is great by the way and easily the best in the series for me personally.
If you're US based the sales on Halo 4 make it a no brainer purchase if you ask me.
As for the multiplayer that's really up to your individual tastes but I'd rate Halo 2/3 ranked a better multiplayer experience overall that lasts longer in terms of re-playability. If you liked Reach's multiplayer then you'll find plenty of playlists you'll like in Halo 4. If you're a CE/2 purist you'll most likely play 1-3 specific Halo 4 playlists.
If you have XBL friends that play Halo 4 you'll have a great time but if you're going in solo the competitive side will be harsh, just as any previous Halo multiplayer. I hope that helps mate.
Separate topic:
What do you all think of the second Gold XBL accounts being purchased to play in the global Halo FFA tourney?
