Stick With Reach
NOTE - I haven't played much Firefight or multi-player game modes like Invasion/Objective games etc, I've stuck to Campaign, Forge and Slayer mostly.
SINGLE PLAYER, SPOILERS
You play as Noble 6, a replacement spartan with a shadowy past, your tasked along with the rest of Noble Team with defending the planet Reach against the Covenant, Reach is the second best defended planet behind Earth and is key to humanity's survival against the Covenant.
First off the campaign doesn't have the best story in the world but at least it's not needlessly confusing like other Halo's and I liked the characters especially Jorge. NOTE to all other developers, Jorge is how you do the big strong guy in a FPS not for example Rico from Killzone, Jorge is hard as nails, strong but doesn't leave his compassionate side behind and this is illustrated well in an early cut scene, I just wish there was more time to get to know Jorge and the other members of Noble Team.
This was definitely the most gritty and realistic Halo game ever, well as realistic as you can get with a game that has enemies like Grunts (their hilarious as ever btw), what im trying to say is that it's campaign really does feel like a military campaign, most objectives feel grounded and relevant like re activating an anti air turret or protecting civilian transports. This is a minor gripe but there seems to be a few instances in the campaign where commanders/soldiers whatever are surprised by massive covenant ships, they seem to appear out of nowhere, you'd think the UNSC would have some way of detecting them but instead they seem to use line of sight. There was even one instance where a human ship flys about a hundred meters away from a covenant structure to destroy it, did it really have to get that close? and then it get's shot from above by a massive covenant ship, again they didn't detect it. Moving on, not having the flood in the game is a major plus, the best thing about Halo 3's campaign was the great enemy AI and you get that same great AI without mindless potato monsters charging at you (The Flood, to me they look like potato's). Friendly AI is well... still shit, your team mates never feel very helpful especially in a vehicle.
As for the general presentation you don't need me to tell you Halo Reach looks great, the character models are top notch and the plasma shot's/lasers whatever are the best in any video game, ever. Running over a hill at the beginning of the game and seeing plasma shots flying at me was amazing especially since I was playing on Heroic and each shot packed a punch although I should mention that the framerate really struggles at certain points. Then I saw a few Elites charge and dodge my grenades and trust me Bungie you made the Elites scary again. The new weapons are mostly great except the focus rifle there's just something I dislike about it, my favourite however is the Grenade Launcher or should I say Pro Tube ehm, it takes a bit of time to master but is very rewarding especially in multi-player. Overall this is the best Halo single player I've played since the original.
Now onto the multi-player (MP) again all I've played is Slayer. Although I have played all the Halo games I haven't spent much time with the MP, played a little Halo 1 on PC, didn't have Xbox Live for Halo 2 and didn't like Halo 3 MP that much but I did play a lot of Halo Reach beta. The core gameplay in Halo Reach fit's well with in the MP like the satisfaction of getting a good rhythm with a DMR/Pistol/Needle Rifle and using the power weapons to their fullest potential. In general I like the bigger emphasis on team play, wearing down shield's before an enemy goes out of view only for your team mate to mop up the kill is more satisfying than any FPS ive played (but I still get pissed when someone "stole my kill" :), never enjoyed getting an assist challenge in a game before. The game modes I have played the most are Slayer varients, not a big fan of SWAT or Snipers though but Im glad they have been given their own playlists. Ive also dipped my toes into the Free for All game modes which are very chaotic but a good way of winding down after a stretch of competitive Slayer matches, Headhunter is currently my favourite, skull collecting sounds like a creepy pastime to anyone unfamiliar with Reach but trust me it's very fun. The leveling up system is great as it takes a lot of credits to go up in rank, I like leveling up systems that take forever, it adds a better sense of accomplishment to each rank, I loved the slow progression in Battlefield 2 and I love it here. Although I haven't unlocked every armour option I would prefer if there were more, maybe hundreds of choices per category, that may be a tall order but it would make your spartan even more unique, hope later DLC adds more.
There are some problems however although some really are just based on perspective, for example im not a big fan of some of the maps such as Asylum, Reflection, Hemorrhage, Zealot or Paradiso. Now I did expect to dislike some maps (cant please everyone) but 5 out of 13 is a bit much, I do suspect that I need to broaden my playlist choices however, I imagine Hemorrhage is a lot more fun with a game type like Big Team Battle rather than 4 v 4 Slayer. The addition of Armour Abilities is great, all of them feel passive, they aid in using tactics to take down an opponent, except Armour Lock which by definition does not aid in tactically taking down an enemy and provides 100% protection from being killed for the time that you've activated it, running/jet packing away from an opponent does not provide 100% protection, luckily I've not run into many people who use it but it is very frustrating when I do.
Another thing that bugs me is you guessed it, the grenades, I think Bungie has come out and said their the same power/blast radius as Halo 3 but they still feel way more effective than Halo 3's, Halo 3's felt pretty useless to me but that was great since I dislike them in all MP games, to me their cheap, grenade spam caused a lot of problems in games like Battlefield 2 and Cod 4 and while their later incarnations addressed the problem Bungie seems to be going the other route, if someone's pacing their shots accurately and the other guy just tosses a grenade then shoots them once and gets the kill or throws one just before he dies to get the kill it creates some un needed frustration, I do it too and I have no problem with others doing it, it's the ability to do it in the first place that bugs me.
It seems like im bashing the MP a bit much but trust me the good far out way the bad and this is the best MP I've played since Cod4, it has a steep learning curve especially if your new to it but it is definetly worth your time and patience to get better at.
Last but certainly not least is Forge, I've spent a lot of time in Forge, certainly more than Halo 3's and that's probably down to the easier to use control options that make making maps a lot more enjoyable. I've made three maps so far and Im in the process of making a fourth. It would have been nice to have a terrain editor to add variety to the maps (their mostly structure based) and some sort of matchmaking such as most played Forge maps but what's here is great and a vast improvement over the original.
To sum up Halo Reach delivers on a great SP and MP experience that I was hoping for and surprised me with how good it's Forge map editor is. It has taken me time to get used to the MP and im getting better and having a lot of fun, as for the single player? well that's just gravy. If you've always loved Halo CE's campaign or the series MP you will arguably have a great time with Reach unless your a purist, if not you probably won't but give it a chance it might surprise you.