Here's some end-game spoilers and my impressions.
So, my impressions? Not nearly as good as Earthbound. The entire last chapter of the game says "Screw You" to Mother 3's plot, and turns the game into an Earthbound spin-off, rather than a sequel. Everything in the game becomes about Mother 2, with the exception of one hallway and a few speeches by some characters, all cobbled together.
Leder's speech, in particular, was incredibly annoying. What's the point of what he tells you? What's the point of the egg? What's the point of the castle. Everything in the game that pertains to Lucas, Claus, Duster, and Kumatora is basically summed up as irrelevant, since Itoi and the plot's developers decided to really reveal things all in one go, by a person that didn't make sense, in order to try to tie up all the loose ends of the plot.
In truth, it became clear the game was about two things: Lucas's family and the end of Porky. In both cases, I felt let down. Since Flint was such a terrible father, I really felt like he had very little place just popping up at the end of the game to finally try to take a little bit of responsibility for Lucas, and only it was convenient to his discovery of Claus. Then, Lucas, rather than growing up and having the guts to end his brother's misery, forces his mostly-brain-dead brother to commit suicide, instead. Essentially, even though the game makes it a point to say Lucas is the coddled baby of the family, he doesn't do much to grow up, aside from set out on adventure, and continue adventuring. He never makes a hard decision. He doesn't fight to protect himself, his friends, or the world. He does exactly the opposite. When it comes time for Lucas to grow up and be a man, he chickens out, and the Deus ex Machina of his dead mother saves the day.
With Porky, we see all the twisted things he's done, but he never really seems to posses the power or greed we saw him growing towards in Mother 2. Honestly, there wasn't a reason to have him in the game, and yet, he took over it. It didn't fit. It didn't fit the nature-based theme, it didn't fit Porky, and it didn't fit the Mother series. Really, in the end, he took a bunch of crazy nutcases from the town of Tazmily and just made them slightly nuttier. I know Earthbound and Mother are supposed to have quirky, weird characters, but the difference between Earthbound and Mother 3's characters is that in Earthbound, the characters know they're flawed, imperfect, and greedy. In Mother 3, they're idiots portrayed as being the last, best people on the planet. They're shallow and superficial from the very start, and unlike in Earthbound, the script isn't quite written as such.
Regardless, if there's one thing I know Mother 3 does too much of, it's carry out a joke for far too long. The oxygen machines come to mind at this point. They were slightly funny the first time. Then jokes about them just kept popping up and kept popping up, and it just wore thin. Likewise, the Magypsies got old pretty quickly, too.
It just felt too scatterbrained and too inconsistent of a game to be on par with Earthbound. Like I said earlier, I think the straw that broke the camel's back was Leder and his speech for me, but there were quite a few things that contributed to what made me value the game much less than Earthbound. Also, Boney should have been better.