It's worth it. Me, my wife, and the three oldest kids have all been using it pretty regularly since October (I think that was when it launched last year).
It will help develop stabilizer muscles, lay a decent foundation of lung-heart, and will build some flexibility. Because the resistance on the ring is fairly low, you aren't going to get massive muscles, though. You can turn the difficulty way up if you want a longer exercise which means more cardio. I use it as a "cooldown" at the end of the day by beating 1 or 2 story levels, about 15-20 minutes in total.
I dunno if it would "get you in shape" but the RPG mechanics are a novel hook, much more interesting than Wii Fit. Picking moves, leveling up, exploiting energy-shakes, upgrading gear, etc etc all feel about as complex as a Mario & Luigi RPG, but instead of timed button-presses you are of course doing an exercise.
As a bonus, the minigames and the moves in general are fun. I hope Nintendo does a sequel and/or more games that utilize the ringcon.