Finished Reach last night and decided to do a short review of the single player portion. 
Spoiler Free review (short):
Overall campaign: 8/10
Gameplay: 8/10
Story: 8/10
Design: 9/10
Music 7/10
Campaign  was a nice change thanks to no flood. Character development was a  little weak. Story was pretty cool. Elites and hunters were a little  overpowered, even on legendary. The music wasn't up to par with the  other games, but was still okay. I beat it on legendary in about 11-12  hours total somewhere in there. Heroic is probably the perfect  difficulty to play it on alone. Legendary is doable, but very  irritating. Good campaign overall and worth it.
Spoiler Free (Long) review and thoughts compared to previous games.
What  was really awesome is that you get to meet Dr. Halsey, who for you  not  too hot on the halo story, is the creator of the Spartan program and   the mind behind of the A.I. Cortana. If you don't know who Cortana is,  then, you probably have no idea what I am talking about. I completed the  game alone on legendary and it was probably just as difficult as Halo CE on legendary.  Halo 2 was way too hard. This is about just right. So do yourself a  favor and just put it on legendary, you'll be glad when you get the  achievement for beating it alone on legendary. A few hair-pulling parts  but nothing that lasted too long. Some parts you just sprint past it all  and get a checkpoint after you clear the area. I had very high hopes  for the music for the game. I'd give it about a 6/10 compared to other  games. Halo CE, 2 and 3 all had great music in the games. ODST's music  did not fit the game well, IMO, ODST was probably the worst Halo game  ever. The campaign can never beat Halo CE's, but considering the fact  that there was no flood, this is the next best campaign next to CE. I  didn't like the so called, "Engineers", if you didn't take them out, it  made the covenant all have about double the shield strength. Elites  didn't die when you stuck them. Their shields didn't completely drain if  you stuck them with a plasma grenade, which is very irritating. Elites  were very mobile and the higher rank elites dove around every time you  tried to shoot them. I liked how they are very hard to kill and were a  real challenge to take down. So when you see a group of 5 you actually  get a little scared. In terms of teammates fighting, they were very  awesome. Noble team actually did something, unlike other halo games. You  really need their help on legendary and it was a great relief to have  them with you. The music. The music felt like it was missing something,  it did incorporate some tunes from the very first game, which was nice.  But that was about the best part of it. Some guitar sequences felt  completely out of balance, while one in particular felt absolutely  perfect. So it was a "oh this is great" to "ahhhhh what?". The majority  of the score worked with the story, just some pieces you can tell it  doesn't go with it. Halo 1, 2, and 3 are all about equally pleasing and  worked much better with the halo story and fit the campaign so much  better. I might have to listen to it again to like it more, but from what I heard, I wasn't as impressive as past games. When I sit down and listen to it without explosions and gunfire, I might have a different opinion. I really wish Bungie would put in audio levels for each component. So you can set how loud the voices, music, gunfire, explosions would be. Some games do it and I love it. It would be pointless for most, but some who love the music or those who want no music and all a battle atmosphere can do it all. But yeah, overall it was a fun experience, I am pretty picky  about music and some character traits that feel off from the story, but  that is just my personal critical eye. If you are a fan of the Halo story, you will not be disappointed by the game. Go rent it and see how the story began. 
The Story(Major Spoilers):
If  you don't want to rent/play it, just read this. The general story is very  basic. The Covenant is attacking the home, training ground and  birthplace of all Spartans. At the very beginning you don't really  understand what is going on until you come face to face with the  Covenant. You meet your fellow Spartans and learn you are replacing a  fallen comrade. You begin to fight and attack in missions in hope to  weaken the attack and save the planet. Up until the last fourth of the game you realize that you are going to lose the battle and plan the last option. A  few missions involved you piloting a spaceship up around the orbital   defense system on Reach and assault a Covenant ship and plant a bomb.  Towards the last few missions you actually experience a Glassing of the planet happen  miles away and are affected by it, which was pretty cool. You run into  Dr. Halsey, as she has special orders for your squad. She has "Cortana"  in a tube and you are chosen by her specifically to take it to Captain  Keyes on the Pillar of Autumn for something unknown that can save  humanity. Fellow Spartans start dying in front of you, most sacrifice  themselves in order for you to complete your mission. In the end, you  deliver the package in time and defend the Autumn from Covenant Phantom  attacks by using this huge "MAC" gun that tears up the sky. You watch the Autumn take off and blast away while you stay back and  fend of the  covenant as the Autumn escapes off. The last sequence is  after the credits, it is a scene that you are actually playing as you  are completely ambushed.  I only lasted about 15 seconds as I was so  confused on what was going  on. When your player runs out of life, it  cuts to a cut scene with your  player as he/she takes off the armor  helmet and the view you are seeing  is the on board camera in the helmet.  You are seen defending yourself  from multiple elites with swords. You  kill a few but are finally  overwhelmed and stabbed to death by a couple  of elites with their  swords. The image you see at the very beginning of  the campaign is the  same image you see at the ending of the campaign.  It is the picture of  your busted helmet that shows your sacrifice and  what you did on planet  Reach. Dr. Halsey is heard giving a prayer to  you wishing you were still  alive to see what you have done and thanks  you for your courage. It was a  pretty emotional ending which leads  perfectly to Halo CE. Then Master Chief comes in and beats some ass and  saves the world, but you know that already.