I saw this the weekend, and I have to say I was disappointed. Only about 1/4 of the movie was in 3D, and the story was quite bland. The visuals were very nice, and Liam Neeson really stole the show as Zeus. Sam Worthington was good as Perseus, but it was the type of role he could have mailed in and it would still suite the film. Definate renter, you aren't missing anything from the 3D. With Avatar just past us, it really let us down in comparison when it comes to 3D.
Check it out if you want a "decent" action flick. Don't go for the 3D.