I've built three of the Lego Mario sets: the starter, the Guarded Fortress and the Bowser's Castle Boss Battle. As builds, they're nothing that exciting. There's plenty that goes into the action features and I've yet to actually try playing it, but it's definitely more for the play experience rather than display. They take up a ton of space, and obviously everything can be connected however you want, including the little 4x4 plates that come with the booster packs.
The real star of the show, for me, is the figures. While Mario is this oversized thing, the enemies are actually pretty decently sized to a minifigure and I'm sure they'll be useful for that one day. For now, once I tear everything down they'll be spread around my display, at least the iconic ones. I don't really care for the modern enemies, but obviously the Koopa, the Goomba, the Piranha Plant and Bowser himself are super classic. I don't have any real underwater sets, but having a Blooper or Cheep Cheep in such a display would be great. If nothing else, they can hang out with my Yellow Submarine. Basically, my Lego collection is a big city packed with random figures, and I love just putting weird combinations of characters in there.
For just the figures, they're super overpriced, but the play features are probably fun for kids and you do get a decent amount of bricks for other stuff, in quite useful colours.