I really like the multiplayer of Halo 5 and I recommend it! Lots of experienced players but nothing that will stop you from doing well ^^
Although the campaign is open to criticism, it is still essential to understanding the overall context of Halo's story so far.
In any case, I wish you a good game!
MP is definitely the biggest draw for H5 at the moment, while there are elite players especially on the ranked playlists, social can be more tolerable at times especial with games like action sack or Fiesta.
Campaign might be essential to understanding the story, but honestly if you start it and realize its not your cup of tea (it wasnt for most players) there are a bunch of recaps on youtube to catch yourself up and the 10 minutes of plot and skip the 8 hours of irrelevant exposition that was H5.