There were scenes when the very crisp picture broke my suspension of disbelief a little - particularly during the scenes with the dwarves and goblins inside the mountains, which somehow had a 'Blue Peter Special' feel about them for me. But those are my least favorite scenes anyway. There were also several points when I found myself noticing how brightly the characters shone out from the distance-blurred background - almost as if they had been shot using a rather old fashioned green screen. In real life, you just can't see individual hairs standing out against a background that is more or less the same tone. It felt odd to be able to pick them out.
That said, there were some scenes where I felt the 3D did enhance the experience - particularly during the scenes at Bag End, when I swear my brain told me that it could smell Gandalf's tobacco smoke drifting into the cinema, before I realised that it must be making that up, and the scene with the Eagles towards the end, where the texture of feathers and the movement of the birds against the sky felt very real.
I greatly enjoyed this variation/retelling, though I'm reserving judgement to some extent until I see the rest.
ETA: have put some spoilers in comments :-D