Reach is Bungie's option on what a really good Halo game is. SPOILER WARNING
Despite the campian story serve as a prequal, Reach is Bungie's Halo 4. It returns to the core of what make Halo:Combat Evolved that damn awesome and evoled again. The intensive campian is the most exciting one ever since Halo 1. The sandboxes are perfect ever being forced to go through the same map twice is no pain in the a**. The story focus on the theme of sacferice as MC on hero and legend and ODST on leave no on behind. This theme really make the campian desprately tensive and it fits the sprit of firefight.
As for compative game, it is better than ever. It's more fast-paced and carries more geekily fun mode. My favorite is one human v. zombie mode called "Safe Haven". The Bungie daily and weekly challenge also make the game never gets old.
If you have a 360 and you heard of Halo you should this, for it is the very game I've been waiting for since I started to track Halo down on E3 2000.