This is an intensely biased statement, but I think you'd have to absolutely hate pure fun to not like New Super Mario Bros Wii. Chris Slate of Nintendo Power made a great statement I agree with:
"While NSMB Wii has largely earned gushing reviews, I've seen a few folks dock points for not being innovative enough or having online play. Huh? Since when is top-notch 2-D Mario action not enough? I'd understand if we got Mario sidescrollers all the time, but this is just the second one in 18 years!"
In regards to no online play, I honestly do not think the experience would be the same. When I've played NSMBWii with my friends and family, there's been so many hilarious and outrageous moments from trying to play multiplayer together. Whether it be someone accidentally taking too many powerups leaving another unprotected, stepping on someone plummeting them to their doom, or simply getting through the level with everyone at the flagpole, all those moments are much more... personal when the other players are sitting right next to you. True, you'd still have that stuff happening, but you wouldn't hear the roars of laughter right next to you, or grin with an evil smile after causing someone to fall off a ledge.
Touching on the nostalgia factor, I did feel like a little kid again when I played World 1-1 and got the Propellor Suit power-up for the first time. But it wasn't just because it was Mario, it was because I was having so much fun.
In addition, this game is actually pretty darn challenging. World 8-1, for example, has a poisonous gas cloud that is constantly scrolling from the left you must run from as molten rocks fall from the sky and spouts of lava gush out of magma pools. Even in the middle worlds, there are some pretty intense spots, in my opinion. And for every moment that gets easier in Multiplayer, there will be ten more moments that are much more harder than they would be in single player. Trust me.
I truly recommend this title to everyone who has a Wii. If not to buy, you should definitely rent it at least once. It's one of those rare games I think almost everyone should play.