Pretty delightful. Story's not much to write home about, but it's delivered with a fluffy energy that's completely charming. The animation is to die for. Whatever Glen Keane (who was originally directing the film) wanted to accomplish with the motion of the characters: I'd say he succeeded with flying colors.
These characters moved with the same sense of energy and freedom we saw in the best of Disney's traditional animation. Alive and hilarious. If you're into that style of animation, you're going to be in heaven. The look and feel of every character was consistently appealing. The film itself is gorgeous. Rapunzel's hair... you're going to want to cut a lock of it to caress as you go to bed.
"It's just you and me now, lock of hair..."
One thing I didn't know about the film going in... it's a musical. A bit of a half-assed one, actually. I think there were only 4 or 5 musical numbers with one of them being a reprise of an earlier song. None of them particularly stood out as anything memorable and I would've rather they hadn't done it as a musical.
A major point against the story: I was bored by the villain. She was never much of a threat (a story problem) and Donna Murphy's voice felt weak. No presence. At least nothing near the presence and delicious villainy of past Disney villains. It felt like the movie stopped cold whenever she was on screen, with the exception of a well-executed creepy moment or two.
Best character: definitely Maximus, the non-giving up horse guy
Moments to savor: a possible homage to Michael Bay/Transformers, another one a homage to Blade Runner, the floating lanterns sequence (gorgeous)
The 3D? It's nice, but certainly not necessary. If you fear for the vibrancy of the film's color palette (it's gorgeous), see it in 2D.