Ah, I dunno. I think you're being a bit overly harsh on the Dune movie by David Lynch. OK, it might have ignored practically all the storyline from the book, but I thought it was very stylishly shot with great visuals and wasn't too bad overall.
I watched Avatar last night in 3D. Possibly the most predictable storyline and most cartoonish characterisation in a film that I've seen in a long time but I enjoyed it enough for the nice visuals. I'll generously award it 6/10. Maybe 6.5/10 at a push.
As an aside, from the two films I've now watched in 3D, I have to say that I don't think it adds a huge amount to the experience. For me, it only really seems to make a difference in slow-panning scenes or where the camera is static when the 3D effect can be taken in properly. For the usual jerky-cam action moments I can't say that I notice much difference between normal footage and 3D.