If you like Halo games, you will probably like Halo: Reach. Early impressions are that it is in fact a Halo game and functions as such.
Halo: Reach is the sixth game in the Halo franchise which, like most video game franchises, stops numbering sequel titles at some arbitrary point and starts inserting random words after a colon instead. So for instance the first sequel was called Halo 2, then there was Halo 3, then they decided to mix things up with Halo 3: ODST. Also there was an RTS called Halo Wars, even though all Halo games are about wars. But we digress.
We don't want to spoil the plot of the single-player mode, though Halo: Reach is a prequel to the original Halo, so if you have played any of the previous games you sort of know the ending (SPOILER: THE GOOD GUYS EVENTUALLY WIN). Let's just say that it involves level after level of shooting increasingly strong aliens using more and more powerful weapons. The plot of the multiplayer is similar, only with the added layer of teenagers vigorously reading from a thesaurus of racial slurs.
The Halo series exists in rare air in the video game industry along with the likes of the Mario, Zelda and Grand Theft Auto franchises, in that the release of a new game grosses the kind of money rarely matched by any product launch--Reach grossed a mind-boggling $200 million its first day. In the middle of a worldwide economic collapse.
The game has received favorable reviews, which is not a surprise as all Halo games are competently made and generally pleasant to play. Reviews also are useless for a franchise like this. If you are a fan of the Halo games, you already own it and in fact are playing it right now instead of reading this. If you dislike Halo games, this one will not change your mind. If you have been resisting trying Halo games through the first five games over the last decade, there is probably no reason to get into them now. You apparently already are satisfied with your life and your other hobbies. You and Halo can continue to coexist in the world without impacting each other's existence in any way.
If you follow the industry, Halo: Reach is significant because it will be the last Halo game developed by Bungie (the studio who created the whole thing). Bungie had made the games exclusively for Microsoft consoles and recently announced it was parting ways with them. However, Microsoft owns Halo, not Bungie, so the split required Bungie to leave their most beloved child behind.
Halo will continue, as Microsoft actually created a studio (343 Industries) to do nothing but focus on future Halo games.
Bungie, meanwhile, jumped from one industry monolith to another: they signed a deal with Activision Blizzard, aka The Largest Video Game Maker That Has Ever Existed, to create a brand new video game series from scratch, which presumably will not involve space marines or armed dune buggies unless they like getting the shit sued out of them.