It's like any exercise routine, if you stick with it, you'll get results. Lots of people who persist with it have reported getting results. I've had Wii Fit (not Plus) and haven't persisted, though it is getting used quite often at our home for the games... but the lack of persistence was primarily because there were no routines and it took time to navigate from one activity to the next.
I've just recently started with EA Sports Active just so I have something to do until I get Wii Fit Plus (which I'll be getting next week). It's ok, and I work up a sweat, but it's less fun than Wii Fit.
To get someone off the couch, your best bet would be Wii Fit Plus, since it'll be so much more fun. And since it now has routines, that may be all they need.
Currently Playing: Shin Megami Tensei: Devil Survivor Overclocked, Professor Layton and the Curious Village
Anticipating: Xenoblade, The Last Story, Mario Kart 7, Rayman Origins, Zelda SS, Crush3D, Tales of the Abyss 3DS, MGS:Snake Eater 3DS, RE:Revelations, Time Travellers, Professor Layton vs. Ace Attorney, Luigi's Mansion 2, MH TriG, DQ Monsters, Heroes of Ruin
