I enjoyed avatar 3d for what it was. It was actually filmed in '3D' (with a lot of cameras at different depths etc), with enormous amounts of work by James Cameron, it took years from what I've read. Getting the timing right down to the single frame and whatnot, I'm glad I don't work in that industry.
Most other '3D' films are, however, filmed in 2D and then sent to Mumbai where all '3D'-esque effects are done in post.
So as most others have said, 3d wont save a bad movie, but done right it might enhance the experience for the viewer. It has done once for me at least (Watching avatar on dvd made me wonder how I could enjoy it the first time), other have been terrible (I left the theater thirty minutes into Burton/Disneys abomination Alice).