For what its worth, I've been using this site for about a year now
ROTTEN TOMATOES: Movies - New Movie Reviews and Previews! I have avoided every shit movie that I might have ordinarily got sucked into ever since, I even use it to rent movies now.
Basically anything
65 - 75% is watchable to decent
76 - 85% is good to very good
85% + is your excellent/superb range
Airbender gets 8%, thats about the lowest rating I've seen. Test it out, pick any classic or any shit film type it in and see what its average % is, its been incredibly consistent for me and have saved me from spending money on dogs like this