Campaign: Overall, it was just "MEH". The campaign was obviously built for die-hard fans and not newcomers. People like me who bought the game as their first halo game often had no idea wtf was going on. It was the most part bland story wise except for a few emotional scenes. After playing reach, I purchased the other halo games (very cheap these days), and Reach's campaign doesn't flow nearly as well as halo 3. It has a nice soundtrack, but, again, not as good as Halo 3's.
Multiplayer: A very enjoyable experience, once you get used to it. Good variety of weapons to choose from, nice selection of vehicles on the larger maps. Many, many different gametypes ranging from free-for-all deathmatches to team-objective based games to big team battles. Playing multiplayer earns you credits which can be spend on new peices of armor for your character, although they only are there for vanity purposes. Your first days will be stressful, everyone already knows where all the good weapons are, and you can expect to take a few one shot kills. My favorite aspect of the multiplayer is the Assasinations, if you sneak up behind the target and hold the melee button, your character will enter a cool animation of usually stabbing the enemy player with his combat knife. Its highly satisfying but can be interrupted.
Forge: A way to build your own maps, you can go into existing maps or the large "Forge world" map and create new structues, item spawns, vehicles, anything you desire. The best community maps are occasionally put into the games map list. Its a fun feature but I haven't gotten too much into it.
Theater: Here you can view replays of all your matches. The cool feature is these replays store the entire match, as in, you can pause at will and see what was happening on the other side of the map then your player character was on. You can also take screenshots and record small clips from these matches and post them to Bungie's website, which is a nice feature.
Meh campaign with enjoyable multiplayer, all in all, I'd say it was worth the 40$ I spent on it.