I saw it just an hour ago and had a great time with it. The script is cheesy at times and the characters are a bit two-dimensional, but all in all it works really well and is a lot of fun, thanks in large part to the actors. Hemsworth especially delivers a great performance and I look forward to seeing him in other Marvel movies. And it's always great to see Natalie Portman, of course.
The one thing that bugged me was the 3D. But I think that had more to do with the calibration of the projector and my position right at the front of the cinema than the 3D conversion (which did feel rather unnecessary, though).
My theater was PACKED, by the way. I hope this is successful enough to warrant further Thor standalone movies.