To me, Halo games on legendary are quite possibly the best gameplay of any games I've ever played. They really really have that 10 seconds of gameplay loop down unlike any game IMO. Halo 5 has this down better than any previous Halo game. No more gimmicks like armor abilities, and instead they added the dash, clamber, and the sprint ability to every Spartan and the game shines unlike any other Halo game before it. Movement is just fun and feels natural. This is the evolution that Halo needed. Keep this for the next game and there really isn't a need to add any gimmicks to add diversity.
With that said, I'm playing through Halo 5 solo on legendary as it's a requirement of mine to do so with every Halo game. I'm on the Sanghelios mission, and all I can say is wow... The missions so far have been fantastic, and Sanghelios is quite possibly up there with the best levels of Halo like Assault on Control Room and Long Night of Solace (no idea if GAF likes this level from Reach, but I loved it).
I've heard plenty of people complaining about the story, but hell, I find it very well done so far. It's living up to the epic nature that Halo games seem to attempt with each release. It even has that sense of wonder which has been missing since Halo 1, with the "Reunion" mission. Forerunner planet with crazy architecture and a very alien yet sublime look. Not knowing where you are and exploring the level brought me back to playing on a Halo and seeing the Halo arc in the sky. That sense of wonder was back.
The game is difficult solo on legendary... The encounters are very well placed and the flow of the game by changing up the enemies and the environment structure are great as well. I can't believe I got past the 1st Warden fight (2nd one was a lot more open and a great fight IMO). I know I'll face him a few more times, and it'll probably frustrate me, but hell, Halo on legendary has never been a cakewalk, and I wouldn't want it any other way.
The only criticism I have so far is the music. At some points it's fantastic. It kicks in at the right time and adds that adrenaline rush. But I've faced a few battles so far where I feel they could've added a track to get the heart pumping, and instead I got no music and just the sound effects. The music there is good, but the 343 Halos really need to look at the Bungie Halos to see how and when to add a track to enhance the mood even more. Bungie's Halos are the best of any game at this IMO. Bring back Marty for Halo 6 and I'll buy the legendary edition to support the decision!
I'm looking forward to beating the campaign and I hope the story doesn't turn into a mess like Halo 2 and Halo 3 did. So far the game's campaign has lived up to the lofty expectations of the Halo franchise. I know this thread has essentially turned into a multi-player only discussion but I just wanted to mention how great the campaign has been so far.