So I got a good deal of time in with the game last night. Played co-op with my wife. First, she had a blast with it, way more than either of us thought she would have. Co-op was the one thing this game was missing and it makes it go from great to phenomenal.
Graphics are nice, no slowdown that I saw. I also never notices slowdown in BotW (other than the random freeze glitch) so take that for what you will. Loading times are very infrequent, and really not that long. It actually gave us enough time to read the little blurbs from Officer Honey, which were quirky, funny, and entertaining.
There do seem to be a few odd quirks with the navigation aid. My wife could not get the navigation studs to appear on her screen, so that might be a P1 thing only, but either player can initiate story sequences and other marker points.
My wife was an awful driver in-game, and coupled with her inability to get the nav guide up she just started riding on my cars. That was funny, but we also discovered that there are some vehicles like buses where the second player can hop on as a passenger.
So I highly recommend this game, even if you already owned it on Wii U. The only people I would caution against this game are Wii U game owners who will never use the co-op mode and people not fond of Lego games (though if you play one, this should be it).