I picked it up for the solo campaign, it's about what I expected. I am about 1/3rd of the way through on Legendary and have yet to really hit a point that took much effort. I think the most time I spent was on a particular point where the only weapon I had was an assault rifle (useless as always) and I had to take down a Wraith and two lads with plasma cannons/sticky grenades. They never really actually make it harder, they just give you less to work with it seems or give you checkpoints in really terrible points in the game.
I've also come across two pretty ridiculous bugs so far, one where your team wont follow you and I had to basically search the level or what to do next and the other where I got stuck on a texture and had to reload from last save points (irritating).
What blows my mind is that after 9 whole years doing Halo games, Bungie STILL cannot write proper team AI to save their lives. If you get into a situation where you are not driving a vehicle and are a passenger or a gunner, you are immediately dead within 10 seconds of someone getting in the drivers seat. Without fail, they drive RIGHT into the middle of a huge swarm of guys that you have no chance of killing. It really isn't that complicated to fix this problem. I ended up having to kill my own team in Halo 3/ODST just to survive and be able to use a vehicle properly. Maybe that's just how it is on Legendary but wtf.