I'd say the co-op is worth it.
In terms of the character selection, P2 is never restricted. They can use any unlocked character and weapon they want, regardless of the story chapter, and regardless of whether or not it's a restricted character mission in Adventure Mode.
As we know, the resolution definitely takes a hit, but if you've ever played a Wii game on a 480p signal, then this should not be unfamiliar to you. It's still perfectly playable. The frame rate will take hits a little more, but it's not that bad, and still perfectly playable.
I kinda wish I had someone to play with me at this point in Adventure Mode, as a lot of the missions are getting too tough for my characters (besides Link and maybe Impa, but they can't unlock everything for me), and just having a second player would make it much more manageable. I could play as the lower level character who as a reward to get in a mission, while my buddy uses Link or Impa to back me up. Also, some of the "Adventure Battles" in Adventure Mode get very hectic, with a lot to manage, so it would be nice to have two people to cover ground.
So yeah if you like co-op games, this one is excellent. Literally the only downside is the resolution thing, and the more likely frame drops, but the former is not unfamiliar to anyone who's ever played a Wii and the game would still look nicer than Twilight Princess, and the latter is barely any worse than what might happen in single-player.
I'd recommend playing through Legend Mode on Normal difficulty, on your own, just to experience the story with the graphics it should be experienced with, and then letting friends join in whenever you're doing the game's various other tasks.