Thor isn't a perfect film. But for people who prefer fantasy over sci-fi, who like character-driven plot as opposed to action, and those who have an interest in mythology, Thor is pretty good. And it's a lovely addition to the Marvel Cinematic Universe.
There were two major issues for me. First, the way Thor's journey to Earth is shown could have been better handled, smaller flashbacks to retain a sense of mystery would be the best and most obvious fix for me. I also felt Loki's descent into villainy could have been more subtle- I felt his plan started too soon, but his character still remained pretty sympathetic. All in all, they managed to squeeze a fair chunk of comic history and Norse legend in one movie, and I think that's a pretty good achievement. An achievement that could have been better done, but I digress.
It is slow in parts and is narmy in places. However, the relationships between the characters are understood and given depth, and the actors do a brilliant job in their portrayals- Loki, Thor and Heimdall especially. Oh, and Darcy. Never forget Darcy. I still felt empathy towards characters, even when they were being stupid or selfish.
It's a fairly entertaining film, but it could have been a really good film. Still, it's fun enough to watch, and the characters are endearing. The acting is good enough that seeing Loki's breakdown is actually quite painful, and seeing Thor become a better person is heartwarming. I'd recommend at least trying it- it isn't everybody's cup of tea, but it's interesting and helps tie up some loose threads of the MCU.